text: // Copyright (c) 2013-2016 Anton Kozhevnikov, Thomas Schulthess
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  /** \file xc_functional.h
   *   
   *  \brief Contains implementation of sirius::XC_functional class.
   */
  
  #ifndef __XC_FUNCTIONAL_H__
  #define __XC_FUNCTIONAL_H__
  
  #include <xc.h>
  #include <string.h>
  #include "utils.h"
  
  namespace sirius {
  
  const std::map<std::string, int> libxc_functionals = {
      {"XC_LDA_X", XC_LDA_X}, /* Exchange */
      {"XC_LDA_C_WIGNER", XC_LDA_C_WIGNER}, /* Wigner parametrization */
      {"XC_LDA_C_RPA", XC_LDA_C_RPA}, /* Random Phase Approximation */
      {"XC_LDA_C_HL", XC_LDA_C_HL}, /* Hedin & Lundqvist */
      {"XC_LDA_C_GL", XC_LDA_C_GL}, /* Gunnarson & Lundqvist */
      {"XC_LDA_C_XALPHA", XC_LDA_C_XALPHA}, /* Slater Xalpha */
      {"XC_LDA_C_VWN", XC_LDA_C_VWN}, /* Vosko, Wilk, & Nusair (5) */
      {"XC_LDA_C_VWN_RPA", XC_LDA_C_VWN_RPA}, /* Vosko, Wilk, & Nusair (RPA) */
      {"XC_LDA_C_PZ", XC_LDA_C_PZ}, /* Perdew & Zunger */
      {"XC_LDA_C_PZ_MOD", XC_LDA_C_PZ_MOD}, /* Perdew & Zunger (Modified) */
      {"XC_LDA_C_OB_PZ", XC_LDA_C_OB_PZ}, /* Ortiz & Ballone (PZ) */
      {"XC_LDA_C_PW", XC_LDA_C_PW}, /* Perdew & Wang */
      {"XC_LDA_C_PW_MOD", XC_LDA_C_PW_MOD}, /* Perdew & Wang (Modified) */
      {"XC_LDA_C_OB_PW", XC_LDA_C_OB_PW}, /* Ortiz & Ballone (PW) */
      {"XC_LDA_C_2D_AMGB", XC_LDA_C_2D_AMGB}, /* Attaccalite et al */
      {"XC_LDA_C_2D_PRM", XC_LDA_C_2D_PRM}, /* Pittalis, Rasanen & Marques correlation in 2D */
      {"XC_LDA_C_vBH", XC_LDA_C_vBH}, /* von Barth & Hedin */
      {"XC_LDA_C_1D_CSC", XC_LDA_C_1D_CSC}, /* Casula, Sorella, and Senatore 1D correlation */
      {"XC_LDA_X_2D", XC_LDA_X_2D}, /* Exchange in 2D */
      {"XC_LDA_XC_TETER93", XC_LDA_XC_TETER93}, /* Teter 93 parametrization */
      {"XC_LDA_X_1D", XC_LDA_X_1D}, /* Exchange in 1D */
      {"XC_LDA_C_ML1", XC_LDA_C_ML1}, /* Modified LSD (version 1) of Proynov and Salahub */
      {"XC_LDA_C_ML2", XC_LDA_C_ML2}, /* Modified LSD (version 2) of Proynov and Salahub */
      {"XC_LDA_C_GOMBAS", XC_LDA_C_GOMBAS}, /* Gombas parametrization */
      {"XC_LDA_C_PW_RPA", XC_LDA_C_PW_RPA}, /* Perdew & Wang fit of the RPA */
      {"XC_LDA_C_1D_LOOS", XC_LDA_C_1D_LOOS}, /* P-F Loos correlation LDA */
      {"XC_LDA_C_RC04", XC_LDA_C_RC04}, /* Ragot-Cortona */
      {"XC_LDA_C_VWN_1", XC_LDA_C_VWN_1}, /* Vosko, Wilk, & Nusair (1) */
      {"XC_LDA_C_VWN_2", XC_LDA_C_VWN_2}, /* Vosko, Wilk, & Nusair (2) */
      {"XC_LDA_C_VWN_3", XC_LDA_C_VWN_3}, /* Vosko, Wilk, & Nusair (3) */
      {"XC_LDA_C_VWN_4", XC_LDA_C_VWN_4}, /* Vosko, Wilk, & Nusair (4) */
      {"XC_LDA_XC_ZLP", XC_LDA_XC_ZLP}, /* Zhao, Levy & Parr, Eq. (20) */
      {"XC_LDA_K_TF", XC_LDA_K_TF}, /* Thomas-Fermi kinetic energy functional */
      {"XC_LDA_K_LP", XC_LDA_K_LP}, /* Lee and Parr Gaussian ansatz */
      {"XC_LDA_XC_KSDT", XC_LDA_XC_KSDT}, /* Karasiev et al. parametrization */
      {"XC_GGA_X_GAM", XC_GGA_X_GAM}, /* GAM functional from Minnesota */
      {"XC_GGA_C_GAM", XC_GGA_C_GAM}, /* GAM functional from Minnesota */
      {"XC_GGA_X_HCTH_A", XC_GGA_X_HCTH_A}, /* HCTH-A */
      {"XC_GGA_X_EV93", XC_GGA_X_EV93}, /* Engel and Vosko */
      {"XC_GGA_X_BGCP", XC_GGA_X_BGCP}, /* Burke, Cancio, Gould, and Pittalis */
      {"XC_GGA_C_BGCP", XC_GGA_C_BGCP}, /* Burke, Cancio, Gould, and Pittalis */
      {"XC_GGA_X_LAMBDA_OC2_N", XC_GGA_X_LAMBDA_OC2_N}, /* lambda_OC2(N) version of PBE */
      {"XC_GGA_X_B86_R", XC_GGA_X_B86_R}, /* Revised Becke 86 Xalpha,beta,gamma (with mod. grad. correction) */
      {"XC_GGA_X_LAMBDA_CH_N", XC_GGA_X_LAMBDA_CH_N}, /* lambda_CH(N) version of PBE */
      {"XC_GGA_X_LAMBDA_LO_N", XC_GGA_X_LAMBDA_LO_N}, /* lambda_LO(N) version of PBE */
      {"XC_GGA_X_HJS_B88_V2", XC_GGA_X_HJS_B88_V2}, /* HJS screened exchange corrected B88 version */
      {"XC_GGA_C_Q2D", XC_GGA_C_Q2D}, /* Chiodo et al */
      {"XC_GGA_X_Q2D", XC_GGA_X_Q2D}, /* Chiodo et al */
      {"XC_GGA_X_PBE_MOL", XC_GGA_X_PBE_MOL}, /* Del Campo, Gazquez, Trickey and Vela (PBE-like) */
      {"XC_GGA_K_TFVW", XC_GGA_K_TFVW}, /* Thomas-Fermi plus von Weiszaecker correction */
      {"XC_GGA_K_REVAPBEINT", XC_GGA_K_REVAPBEINT}, /* interpolated version of REVAPBE */
      {"XC_GGA_K_APBEINT", XC_GGA_K_APBEINT}, /* interpolated version of APBE */
      {"XC_GGA_K_REVAPBE", XC_GGA_K_REVAPBE}, /* revised APBE */
      {"XC_GGA_X_AK13", XC_GGA_X_AK13}, /* Armiento & Kuemmel 2013 */
      {"XC_GGA_K_MEYER", XC_GGA_K_MEYER}, /* Meyer,  Wang, and Young */
      {"XC_GGA_X_LV_RPW86", XC_GGA_X_LV_RPW86}, /* Berland and Hyldgaard */
      {"XC_GGA_X_PBE_TCA", XC_GGA_X_PBE_TCA}, /* PBE revised by Tognetti et al */
      {"XC_GGA_X_PBEINT", XC_GGA_X_PBEINT}, /* PBE for hybrid interfaces */
      {"XC_GGA_C_ZPBEINT", XC_GGA_C_ZPBEINT}, /* spin-dependent gradient correction to PBEint */
      {"XC_GGA_C_PBEINT", XC_GGA_C_PBEINT}, /* PBE for hybrid interfaces */
      {"XC_GGA_C_ZPBESOL", XC_GGA_C_ZPBESOL}, /* spin-dependent gradient correction to PBEsol */
      {"XC_GGA_XC_OPBE_D", XC_GGA_XC_OPBE_D}, /* oPBE_D functional of Goerigk and Grimme */
      {"XC_GGA_XC_OPWLYP_D", XC_GGA_XC_OPWLYP_D}, /* oPWLYP-D functional of Goerigk and Grimme */
      {"XC_GGA_XC_OBLYP_D", XC_GGA_XC_OBLYP_D}, /* oBLYP-D functional of Goerigk and Grimme */
      {"XC_GGA_X_VMT84_GE", XC_GGA_X_VMT84_GE}, /* VMT{8,4} with constraint satisfaction with mu = mu_GE */
      {"XC_GGA_X_VMT84_PBE", XC_GGA_X_VMT84_PBE}, /* VMT{8,4} with constraint satisfaction with mu = mu_PBE */
      {"XC_GGA_X_VMT_GE", XC_GGA_X_VMT_GE}, /* Vela, Medel, and Trickey with mu = mu_GE */
      {"XC_GGA_X_VMT_PBE", XC_GGA_X_VMT_PBE}, /* Vela, Medel, and Trickey with mu = mu_PBE */
      {"XC_GGA_C_N12_SX", XC_GGA_C_N12_SX}, /* N12-SX functional from Minnesota */
      {"XC_GGA_C_N12", XC_GGA_C_N12}, /* N12 functional from Minnesota */
      {"XC_GGA_X_N12", XC_GGA_X_N12}, /* N12 functional from Minnesota */
      {"XC_GGA_C_REGTPSS", XC_GGA_C_REGTPSS}, /* Regularized TPSS correlation (ex-VPBE) */
      {"XC_GGA_C_OP_XALPHA", XC_GGA_C_OP_XALPHA}, /* one-parameter progressive functional (XALPHA version) */
      {"XC_GGA_C_OP_G96", XC_GGA_C_OP_G96}, /* one-parameter progressive functional (G96 version) */
      {"XC_GGA_C_OP_PBE", XC_GGA_C_OP_PBE}, /* one-parameter progressive functional (PBE version) */
      {"XC_GGA_C_OP_B88", XC_GGA_C_OP_B88}, /* one-parameter progressive functional (B88 version) */
      {"XC_GGA_C_FT97", XC_GGA_C_FT97}, /* Filatov & Thiel correlation */
      {"XC_GGA_C_SPBE", XC_GGA_C_SPBE}, /* PBE correlation to be used with the SSB exchange */
      {"XC_GGA_X_SSB_SW", XC_GGA_X_SSB_SW}, /* Swarta, Sola and Bickelhaupt correction to PBE */
      {"XC_GGA_X_SSB", XC_GGA_X_SSB}, /* Swarta, Sola and Bickelhaupt */
      {"XC_GGA_X_SSB_D", XC_GGA_X_SSB_D}, /* Swarta, Sola and Bickelhaupt dispersion */
      {"XC_GGA_XC_HCTH_407P", XC_GGA_XC_HCTH_407P}, /* HCTH/407+ */
      {"XC_GGA_XC_HCTH_P76", XC_GGA_XC_HCTH_P76}, /* HCTH p=7/6 */
      {"XC_GGA_XC_HCTH_P14", XC_GGA_XC_HCTH_P14}, /* HCTH p=1/4 */
      {"XC_GGA_XC_B97_GGA1", XC_GGA_XC_B97_GGA1}, /* Becke 97 GGA-1 */
      {"XC_GGA_C_HCTH_A", XC_GGA_C_HCTH_A}, /* HCTH-A */
      {"XC_GGA_X_BPCCAC", XC_GGA_X_BPCCAC}, /* BPCCAC (GRAC for the energy) */
      {"XC_GGA_C_REVTCA", XC_GGA_C_REVTCA}, /* Tognetti, Cortona, Adamo (revised) */
      {"XC_GGA_C_TCA", XC_GGA_C_TCA}, /* Tognetti, Cortona, Adamo */
      {"XC_GGA_X_PBE", XC_GGA_X_PBE}, /* Perdew, Burke & Ernzerhof exchange */
      {"XC_GGA_X_PBE_R", XC_GGA_X_PBE_R}, /* Perdew, Burke & Ernzerhof exchange (revised) */
      {"XC_GGA_X_B86", XC_GGA_X_B86}, /* Becke 86 Xalpha,beta,gamma */
      {"XC_GGA_X_HERMAN", XC_GGA_X_HERMAN}, /* Herman et al original GGA */
      {"XC_GGA_X_B86_MGC", XC_GGA_X_B86_MGC}, /* Becke 86 Xalpha,beta,gamma (with mod. grad. correction) */
      {"XC_GGA_X_B88", XC_GGA_X_B88}, /* Becke 88 */
      {"XC_GGA_X_G96", XC_GGA_X_G96}, /* Gill 96 */
      {"XC_GGA_X_PW86", XC_GGA_X_PW86}, /* Perdew & Wang 86 */
      {"XC_GGA_X_PW91", XC_GGA_X_PW91}, /* Perdew & Wang 91 */
      {"XC_GGA_X_OPTX", XC_GGA_X_OPTX}, /* Handy & Cohen OPTX 01 */
      {"XC_GGA_X_DK87_R1", XC_GGA_X_DK87_R1}, /* dePristo & Kress 87 (version R1) */
      {"XC_GGA_X_DK87_R2", XC_GGA_X_DK87_R2}, /* dePristo & Kress 87 (version R2) */
      {"XC_GGA_X_LG93", XC_GGA_X_LG93}, /* Lacks & Gordon 93 */
      {"XC_GGA_X_FT97_A", XC_GGA_X_FT97_A}, /* Filatov & Thiel 97 (version A) */
      {"XC_GGA_X_FT97_B", XC_GGA_X_FT97_B}, /* Filatov & Thiel 97 (version B) */
      {"XC_GGA_X_PBE_SOL", XC_GGA_X_PBE_SOL}, /* Perdew, Burke & Ernzerhof exchange (solids) */
      {"XC_GGA_X_RPBE", XC_GGA_X_RPBE}, /* Hammer, Hansen & Norskov (PBE-like) */
      {"XC_GGA_X_WC", XC_GGA_X_WC}, /* Wu & Cohen */
      {"XC_GGA_X_MPW91", XC_GGA_X_MPW91}, /* Modified form of PW91 by Adamo & Barone */
      {"XC_GGA_X_AM05", XC_GGA_X_AM05}, /* Armiento & Mattsson 05 exchange */
      {"XC_GGA_X_PBEA", XC_GGA_X_PBEA}, /* Madsen (PBE-like) */
      {"XC_GGA_X_MPBE", XC_GGA_X_MPBE}, /* Adamo & Barone modification to PBE */
      {"XC_GGA_X_XPBE", XC_GGA_X_XPBE}, /* xPBE reparametrization by Xu & Goddard */
      {"XC_GGA_X_2D_B86_MGC", XC_GGA_X_2D_B86_MGC}, /* Becke 86 MGC for 2D systems */
      {"XC_GGA_X_BAYESIAN", XC_GGA_X_BAYESIAN}, /* Bayesian best fit for the enhancement factor */
      {"XC_GGA_X_PBE_JSJR", XC_GGA_X_PBE_JSJR}, /* JSJR reparametrization by Pedroza, Silva & Capelle */
      {"XC_GGA_X_2D_B88", XC_GGA_X_2D_B88}, /* Becke 88 in 2D */
      {"XC_GGA_X_2D_B86", XC_GGA_X_2D_B86}, /* Becke 86 Xalpha,beta,gamma */
      {"XC_GGA_X_2D_PBE", XC_GGA_X_2D_PBE}, /* Perdew, Burke & Ernzerhof exchange in 2D */
      {"XC_GGA_C_PBE", XC_GGA_C_PBE}, /* Perdew, Burke & Ernzerhof correlation */
      {"XC_GGA_C_LYP", XC_GGA_C_LYP}, /* Lee, Yang & Parr */
      {"XC_GGA_C_P86", XC_GGA_C_P86}, /* Perdew 86 */
      {"XC_GGA_C_PBE_SOL", XC_GGA_C_PBE_SOL}, /* Perdew, Burke & Ernzerhof correlation SOL */
      {"XC_GGA_C_PW91", XC_GGA_C_PW91}, /* Perdew & Wang 91 */
      {"XC_GGA_C_AM05", XC_GGA_C_AM05}, /* Armiento & Mattsson 05 correlation */
      {"XC_GGA_C_XPBE", XC_GGA_C_XPBE}, /* xPBE reparametrization by Xu & Goddard */
      {"XC_GGA_C_LM", XC_GGA_C_LM}, /* Langreth and Mehl correlation */
      {"XC_GGA_C_PBE_JRGX", XC_GGA_C_PBE_JRGX}, /* JRGX reparametrization by Pedroza, Silva & Capelle */
      {"XC_GGA_X_OPTB88_VDW", XC_GGA_X_OPTB88_VDW}, /* Becke 88 reoptimized to be used with vdW functional of Dion et al */
      {"XC_GGA_X_PBEK1_VDW", XC_GGA_X_PBEK1_VDW}, /* PBE reparametrization for vdW */
      {"XC_GGA_X_OPTPBE_VDW", XC_GGA_X_OPTPBE_VDW}, /* PBE reparametrization for vdW */
      {"XC_GGA_X_RGE2", XC_GGA_X_RGE2}, /* Regularized PBE */
      {"XC_GGA_C_RGE2", XC_GGA_C_RGE2}, /* Regularized PBE */
      {"XC_GGA_X_RPW86", XC_GGA_X_RPW86}, /* refitted Perdew & Wang 86 */
      {"XC_GGA_X_KT1", XC_GGA_X_KT1}, /* Keal and Tozer version 1 */
      {"XC_GGA_XC_KT2", XC_GGA_XC_KT2}, /* Keal and Tozer version 2 */
      {"XC_GGA_C_WL", XC_GGA_C_WL}, /* Wilson & Levy */
      {"XC_GGA_C_WI", XC_GGA_C_WI}, /* Wilson & Ivanov */
      {"XC_GGA_X_MB88", XC_GGA_X_MB88}, /* Modified Becke 88 for proton transfer */
      {"XC_GGA_X_SOGGA", XC_GGA_X_SOGGA}, /* Second-order generalized gradient approximation */
      {"XC_GGA_X_SOGGA11", XC_GGA_X_SOGGA11}, /* Second-order generalized gradient approximation 2011 */
      {"XC_GGA_C_SOGGA11", XC_GGA_C_SOGGA11}, /* Second-order generalized gradient approximation 2011 */
      {"XC_GGA_C_WI0", XC_GGA_C_WI0}, /* Wilson & Ivanov initial version */
      {"XC_GGA_XC_TH1", XC_GGA_XC_TH1}, /* Tozer and Handy v. 1 */
      {"XC_GGA_XC_TH2", XC_GGA_XC_TH2}, /* Tozer and Handy v. 2 */
      {"XC_GGA_XC_TH3", XC_GGA_XC_TH3}, /* Tozer and Handy v. 3 */
      {"XC_GGA_XC_TH4", XC_GGA_XC_TH4}, /* Tozer and Handy v. 4 */
      {"XC_GGA_X_C09X", XC_GGA_X_C09X}, /* C09x to be used with the VdW of Rutgers-Chalmers */
      {"XC_GGA_C_SOGGA11_X", XC_GGA_C_SOGGA11_X}, /* To be used with HYB_GGA_X_SOGGA11_X */
      {"XC_GGA_X_LB", XC_GGA_X_LB}, /* van Leeuwen & Baerends */
      {"XC_GGA_XC_HCTH_93", XC_GGA_XC_HCTH_93}, /* HCTH functional fitted to  93 molecules */
      {"XC_GGA_XC_HCTH_120", XC_GGA_XC_HCTH_120}, /* HCTH functional fitted to 120 molecules */
      {"XC_GGA_XC_HCTH_147", XC_GGA_XC_HCTH_147}, /* HCTH functional fitted to 147 molecules */
      {"XC_GGA_XC_HCTH_407", XC_GGA_XC_HCTH_407}, /* HCTH functional fitted to 407 molecules */
      {"XC_GGA_XC_EDF1", XC_GGA_XC_EDF1}, /* Empirical functionals from Adamson, Gill, and Pople */
      {"XC_GGA_XC_XLYP", XC_GGA_XC_XLYP}, /* XLYP functional */
      {"XC_GGA_XC_B97_D", XC_GGA_XC_B97_D}, /* Grimme functional to be used with C6 vdW term */
      {"XC_GGA_XC_PBE1W", XC_GGA_XC_PBE1W}, /* Functionals fitted for water */
      {"XC_GGA_XC_MPWLYP1W", XC_GGA_XC_MPWLYP1W}, /* Functionals fitted for water */
      {"XC_GGA_XC_PBELYP1W", XC_GGA_XC_PBELYP1W}, /* Functionals fitted for water */
      {"XC_GGA_X_LBM", XC_GGA_X_LBM}, /* van Leeuwen & Baerends modified */
      {"XC_GGA_X_OL2", XC_GGA_X_OL2}, /* Exchange form based on Ou-Yang and Levy v.2 */
      {"XC_GGA_X_APBE", XC_GGA_X_APBE}, /* mu fixed from the semiclassical neutral atom */
      {"XC_GGA_K_APBE", XC_GGA_K_APBE}, /* mu fixed from the semiclassical neutral atom */
      {"XC_GGA_C_APBE", XC_GGA_C_APBE}, /* mu fixed from the semiclassical neutral atom */
      {"XC_GGA_K_TW1", XC_GGA_K_TW1}, /* Tran and Wesolowski set 1 (Table II) */
      {"XC_GGA_K_TW2", XC_GGA_K_TW2}, /* Tran and Wesolowski set 2 (Table II) */
      {"XC_GGA_K_TW3", XC_GGA_K_TW3}, /* Tran and Wesolowski set 3 (Table II) */
      {"XC_GGA_K_TW4", XC_GGA_K_TW4}, /* Tran and Wesolowski set 4 (Table II) */
      {"XC_GGA_X_HTBS", XC_GGA_X_HTBS}, /* Haas, Tran, Blaha, and Schwarz */
      {"XC_GGA_X_AIRY", XC_GGA_X_AIRY}, /* Constantin et al based on the Airy gas */
      {"XC_GGA_X_LAG", XC_GGA_X_LAG}, /* Local Airy Gas */
      {"XC_GGA_XC_MOHLYP", XC_GGA_XC_MOHLYP}, /* Functional for organometallic chemistry */
      {"XC_GGA_XC_MOHLYP2", XC_GGA_XC_MOHLYP2}, /* Functional for barrier heights */
      {"XC_GGA_XC_TH_FL", XC_GGA_XC_TH_FL}, /* Tozer and Handy v. FL */
      {"XC_GGA_XC_TH_FC", XC_GGA_XC_TH_FC}, /* Tozer and Handy v. FC */
      {"XC_GGA_XC_TH_FCFO", XC_GGA_XC_TH_FCFO}, /* Tozer and Handy v. FCFO */
      {"XC_GGA_XC_TH_FCO", XC_GGA_XC_TH_FCO}, /* Tozer and Handy v. FCO */
      {"XC_GGA_C_OPTC", XC_GGA_C_OPTC}, /* Optimized correlation functional of Cohen and Handy */
      {"XC_GGA_C_PBELOC", XC_GGA_C_PBELOC}, /* Semilocal dynamical correlation */
      {"XC_GGA_XC_VV10", XC_GGA_XC_VV10}, /* Vydrov and Van Voorhis */
      {"XC_GGA_C_PBEFE", XC_GGA_C_PBEFE}, /* PBE for formation energies */
      {"XC_GGA_C_OP_PW91", XC_GGA_C_OP_PW91}, /* one-parameter progressive functional (PW91 version) */
      {"XC_GGA_X_PBEFE", XC_GGA_X_PBEFE}, /* PBE for formation energies */
      {"XC_GGA_X_CAP", XC_GGA_X_CAP}, /* Correct Asymptotic Potential */
      {"XC_GGA_K_VW", XC_GGA_K_VW}, /* von Weiszaecker functional */
      {"XC_GGA_K_GE2", XC_GGA_K_GE2}, /* Second-order gradient expansion (l = 1/9) */
      {"XC_GGA_K_GOLDEN", XC_GGA_K_GOLDEN}, /* TF-lambda-vW form by Golden (l = 13/45) */
      {"XC_GGA_K_YT65", XC_GGA_K_YT65}, /* TF-lambda-vW form by Yonei and Tomishima (l = 1/5) */
      {"XC_GGA_K_BALTIN", XC_GGA_K_BALTIN}, /* TF-lambda-vW form by Baltin (l = 5/9) */
      {"XC_GGA_K_LIEB", XC_GGA_K_LIEB}, /* TF-lambda-vW form by Lieb (l = 0.185909191) */
      {"XC_GGA_K_ABSP1", XC_GGA_K_ABSP1}, /* gamma-TFvW form by Acharya et al [g = 1 - 1.412/N^(1/3)] */
      {"XC_GGA_K_ABSP2", XC_GGA_K_ABSP2}, /* gamma-TFvW form by Acharya et al [g = 1 - 1.332/N^(1/3)] */
      {"XC_GGA_K_GR", XC_GGA_K_GR}, /* gamma-TFvW form by Gazquez and Robles */
      {"XC_GGA_K_LUDENA", XC_GGA_K_LUDENA}, /* gamma-TFvW form by Ludena */
      {"XC_GGA_K_GP85", XC_GGA_K_GP85}, /* gamma-TFvW form by Ghosh and Parr */
      {"XC_GGA_K_PEARSON", XC_GGA_K_PEARSON}, /* Pearson */
      {"XC_GGA_K_OL1", XC_GGA_K_OL1}, /* Ou-Yang and Levy v.1 */
      {"XC_GGA_K_OL2", XC_GGA_K_OL2}, /* Ou-Yang and Levy v.2 */
      {"XC_GGA_K_FR_B88", XC_GGA_K_FR_B88}, /* Fuentealba & Reyes (B88 version) */
      {"XC_GGA_K_FR_PW86", XC_GGA_K_FR_PW86}, /* Fuentealba & Reyes (PW86 version) */
      {"XC_GGA_K_DK", XC_GGA_K_DK}, /* DePristo and Kress */
      {"XC_GGA_K_PERDEW", XC_GGA_K_PERDEW}, /* Perdew */
      {"XC_GGA_K_VSK", XC_GGA_K_VSK}, /* Vitos, Skriver, and Kollar */
      {"XC_GGA_K_VJKS", XC_GGA_K_VJKS}, /* Vitos, Johansson, Kollar, and Skriver */
      {"XC_GGA_K_ERNZERHOF", XC_GGA_K_ERNZERHOF}, /* Ernzerhof */
      {"XC_GGA_K_LC94", XC_GGA_K_LC94}, /* Lembarki & Chermette */
      {"XC_GGA_K_LLP", XC_GGA_K_LLP}, /* Lee, Lee & Parr */
      {"XC_GGA_K_THAKKAR", XC_GGA_K_THAKKAR}, /* Thakkar 1992 */
      {"XC_GGA_X_WPBEH", XC_GGA_X_WPBEH}, /* short-range version of the PBE */
      {"XC_GGA_X_HJS_PBE", XC_GGA_X_HJS_PBE}, /* HJS screened exchange PBE version */
      {"XC_GGA_X_HJS_PBE_SOL", XC_GGA_X_HJS_PBE_SOL}, /* HJS screened exchange PBE_SOL version */
      {"XC_GGA_X_HJS_B88", XC_GGA_X_HJS_B88}, /* HJS screened exchange B88 version */
      {"XC_GGA_X_HJS_B97X", XC_GGA_X_HJS_B97X}, /* HJS screened exchange B97x version */
      {"XC_GGA_X_ITYH", XC_GGA_X_ITYH}, /* short-range recipe for exchange GGA functionals */
      {"XC_GGA_X_SFAT", XC_GGA_X_SFAT}, /* short-range recipe for exchange GGA functionals */
      {"XC_HYB_GGA_X_N12_SX", XC_HYB_GGA_X_N12_SX}, /* N12-SX functional from Minnesota */
      {"XC_HYB_GGA_XC_B97_1p", XC_HYB_GGA_XC_B97_1p}, /* version of B97 by Cohen and Handy */
      {"XC_HYB_GGA_XC_B3PW91", XC_HYB_GGA_XC_B3PW91}, /* The original (ACM) hybrid of Becke */
      {"XC_HYB_GGA_XC_B3LYP", XC_HYB_GGA_XC_B3LYP}, /* The (in)famous B3LYP */
      {"XC_HYB_GGA_XC_B3P86", XC_HYB_GGA_XC_B3P86}, /* Perdew 86 hybrid similar to B3PW91 */
      {"XC_HYB_GGA_XC_O3LYP", XC_HYB_GGA_XC_O3LYP}, /* hybrid using the optx functional */
      {"XC_HYB_GGA_XC_mPW1K", XC_HYB_GGA_XC_mPW1K}, /* mixture of mPW91 and PW91 optimized for kinetics */
      {"XC_HYB_GGA_XC_PBEH", XC_HYB_GGA_XC_PBEH}, /* aka PBE0 or PBE1PBE */
      {"XC_HYB_GGA_XC_B97", XC_HYB_GGA_XC_B97}, /* Becke 97 */
      {"XC_HYB_GGA_XC_B97_1", XC_HYB_GGA_XC_B97_1}, /* Becke 97-1 */
      {"XC_HYB_GGA_XC_B97_2", XC_HYB_GGA_XC_B97_2}, /* Becke 97-2 */
      {"XC_HYB_GGA_XC_X3LYP", XC_HYB_GGA_XC_X3LYP}, /* hybrid by Xu and Goddard */
      {"XC_HYB_GGA_XC_B1WC", XC_HYB_GGA_XC_B1WC}, /* Becke 1-parameter mixture of WC and PBE */
      {"XC_HYB_GGA_XC_B97_K", XC_HYB_GGA_XC_B97_K}, /* Boese-Martin for Kinetics */
      {"XC_HYB_GGA_XC_B97_3", XC_HYB_GGA_XC_B97_3}, /* Becke 97-3 */
      {"XC_HYB_GGA_XC_MPW3PW", XC_HYB_GGA_XC_MPW3PW}, /* mixture with the mPW functional */
      {"XC_HYB_GGA_XC_B1LYP", XC_HYB_GGA_XC_B1LYP}, /* Becke 1-parameter mixture of B88 and LYP */
      {"XC_HYB_GGA_XC_B1PW91", XC_HYB_GGA_XC_B1PW91}, /* Becke 1-parameter mixture of B88 and PW91 */
      {"XC_HYB_GGA_XC_mPW1PW", XC_HYB_GGA_XC_mPW1PW}, /* Becke 1-parameter mixture of mPW91 and PW91 */
      {"XC_HYB_GGA_XC_MPW3LYP", XC_HYB_GGA_XC_MPW3LYP}, /* mixture of mPW and LYP */
      {"XC_HYB_GGA_XC_SB98_1a", XC_HYB_GGA_XC_SB98_1a}, /* Schmider-Becke 98 parameterization 1a */
      {"XC_HYB_GGA_XC_SB98_1b", XC_HYB_GGA_XC_SB98_1b}, /* Schmider-Becke 98 parameterization 1b */
      {"XC_HYB_GGA_XC_SB98_1c", XC_HYB_GGA_XC_SB98_1c}, /* Schmider-Becke 98 parameterization 1c */
      {"XC_HYB_GGA_XC_SB98_2a", XC_HYB_GGA_XC_SB98_2a}, /* Schmider-Becke 98 parameterization 2a */
      {"XC_HYB_GGA_XC_SB98_2b", XC_HYB_GGA_XC_SB98_2b}, /* Schmider-Becke 98 parameterization 2b */
      {"XC_HYB_GGA_XC_SB98_2c", XC_HYB_GGA_XC_SB98_2c}, /* Schmider-Becke 98 parameterization 2c */
      {"XC_HYB_GGA_X_SOGGA11_X", XC_HYB_GGA_X_SOGGA11_X}, /* Hybrid based on SOGGA11 form */
      {"XC_HYB_GGA_XC_HSE03", XC_HYB_GGA_XC_HSE03}, /* the 2003 version of the screened hybrid HSE */
      {"XC_HYB_GGA_XC_HSE06", XC_HYB_GGA_XC_HSE06}, /* the 2006 version of the screened hybrid HSE */
      {"XC_HYB_GGA_XC_HJS_PBE", XC_HYB_GGA_XC_HJS_PBE}, /* HJS hybrid screened exchange PBE version */
      {"XC_HYB_GGA_XC_HJS_PBE_SOL", XC_HYB_GGA_XC_HJS_PBE_SOL}, /* HJS hybrid screened exchange PBE_SOL version */
      {"XC_HYB_GGA_XC_HJS_B88", XC_HYB_GGA_XC_HJS_B88}, /* HJS hybrid screened exchange B88 version */
      {"XC_HYB_GGA_XC_HJS_B97X", XC_HYB_GGA_XC_HJS_B97X}, /* HJS hybrid screened exchange B97x version */
      {"XC_HYB_GGA_XC_CAM_B3LYP", XC_HYB_GGA_XC_CAM_B3LYP}, /* CAM version of B3LYP */
      {"XC_HYB_GGA_XC_TUNED_CAM_B3LYP", XC_HYB_GGA_XC_TUNED_CAM_B3LYP}, /* CAM version of B3LYP tuned for excitations */
      {"XC_HYB_GGA_XC_BHANDH", XC_HYB_GGA_XC_BHANDH}, /* Becke half-and-half */
      {"XC_HYB_GGA_XC_BHANDHLYP", XC_HYB_GGA_XC_BHANDHLYP}, /* Becke half-and-half with B88 exchange */
      {"XC_HYB_GGA_XC_MB3LYP_RC04", XC_HYB_GGA_XC_MB3LYP_RC04}, /* B3LYP with RC04 LDA */
      {"XC_HYB_GGA_XC_MPWLYP1M", XC_HYB_GGA_XC_MPWLYP1M}, /* MPW with 1 par. for metals/LYP */
      {"XC_HYB_GGA_XC_REVB3LYP", XC_HYB_GGA_XC_REVB3LYP}, /* Revised B3LYP */
      {"XC_HYB_GGA_XC_CAMY_BLYP", XC_HYB_GGA_XC_CAMY_BLYP}, /* BLYP with yukawa screening */
      {"XC_HYB_GGA_XC_PBE0_13", XC_HYB_GGA_XC_PBE0_13}, /* PBE0-1/3 */
      {"XC_HYB_GGA_XC_B3LYPs", XC_HYB_GGA_XC_B3LYPs}, /* B3LYP * functional */
      {"XC_HYB_GGA_XC_WB97", XC_HYB_GGA_XC_WB97}, /* Chai and Head-Gordon */
      {"XC_HYB_GGA_XC_WB97X", XC_HYB_GGA_XC_WB97X}, /* Chai and Head-Gordon */
      {"XC_HYB_GGA_XC_LRC_WPBEH", XC_HYB_GGA_XC_LRC_WPBEH}, /* Long-range corrected functional by Rorhdanz et al */
      {"XC_HYB_GGA_XC_WB97X_V", XC_HYB_GGA_XC_WB97X_V}, /* Mardirossian and Head-Gordon */
      {"XC_HYB_GGA_XC_LCY_PBE", XC_HYB_GGA_XC_LCY_PBE}, /* PBE with yukawa screening */
      {"XC_HYB_GGA_XC_LCY_BLYP", XC_HYB_GGA_XC_LCY_BLYP}, /* BLYP with yukawa screening */
      {"XC_HYB_GGA_XC_LC_VV10", XC_HYB_GGA_XC_LC_VV10}, /* Vydrov and Van Voorhis */
      {"XC_HYB_GGA_XC_CAMY_B3LYP", XC_HYB_GGA_XC_CAMY_B3LYP}, /* B3LYP with Yukawa screening */
      {"XC_HYB_GGA_XC_WB97X_D", XC_HYB_GGA_XC_WB97X_D}, /* Chai and Head-Gordon */
      {"XC_HYB_GGA_XC_HPBEINT", XC_HYB_GGA_XC_HPBEINT}, /* hPBEint */
      {"XC_HYB_GGA_XC_LRC_WPBE", XC_HYB_GGA_XC_LRC_WPBE}, /* Long-range corrected functional by Rorhdanz et al */
      {"XC_HYB_GGA_XC_B3LYP5", XC_HYB_GGA_XC_B3LYP5}, /* B3LYP with VWN functional 5 instead of RPA */
      {"XC_HYB_GGA_XC_EDF2", XC_HYB_GGA_XC_EDF2}, /* Empirical functional from Lin, George and Gill */
      {"XC_HYB_GGA_XC_CAP0", XC_HYB_GGA_XC_CAP0}, /* Correct Asymptotic Potential hybrid */
      {"XC_MGGA_C_DLDF", XC_MGGA_C_DLDF}, /* Dispersionless Density Functional */
      {"XC_MGGA_XC_ZLP", XC_MGGA_XC_ZLP}, /* Zhao, Levy & Parr, Eq. (21) */
      {"XC_MGGA_XC_OTPSS_D", XC_MGGA_XC_OTPSS_D}, /* oTPSS_D functional of Goerigk and Grimme */
      {"XC_MGGA_C_CS", XC_MGGA_C_CS}, /* Colle and Salvetti */
      {"XC_MGGA_C_MN12_SX", XC_MGGA_C_MN12_SX}, /* Worker for MN12-SX functional */
      {"XC_MGGA_C_MN12_L", XC_MGGA_C_MN12_L}, /* MN12-L functional from Minnesota */
      {"XC_MGGA_C_M11_L", XC_MGGA_C_M11_L}, /* M11-L functional from Minnesota */
      {"XC_MGGA_C_M11", XC_MGGA_C_M11}, /* Worker for M11 functional */
      {"XC_MGGA_C_M08_SO", XC_MGGA_C_M08_SO}, /* Worker for M08-SO functional */
      {"XC_MGGA_C_M08_HX", XC_MGGA_C_M08_HX}, /* Worker for M08-HX functional */
      {"XC_MGGA_X_LTA", XC_MGGA_X_LTA}, /* Local tau approximation of Ernzerhof & Scuseria */
      {"XC_MGGA_X_TPSS", XC_MGGA_X_TPSS}, /* Perdew, Tao, Staroverov & Scuseria exchange */
      {"XC_MGGA_X_M06_L", XC_MGGA_X_M06_L}, /* M06-Local functional of Minnesota */
      {"XC_MGGA_X_GVT4", XC_MGGA_X_GVT4}, /* GVT4 from Van Voorhis and Scuseria */
      {"XC_MGGA_X_TAU_HCTH", XC_MGGA_X_TAU_HCTH}, /* tau-HCTH from Boese and Handy */
      {"XC_MGGA_X_BR89", XC_MGGA_X_BR89}, /* Becke-Roussel 89 */
      {"XC_MGGA_X_BJ06", XC_MGGA_X_BJ06}, /* Becke & Johnson correction to Becke-Roussel 89 */
      {"XC_MGGA_X_TB09", XC_MGGA_X_TB09}, /* Tran & Blaha correction to Becke & Johnson */
      {"XC_MGGA_X_RPP09", XC_MGGA_X_RPP09}, /* Rasanen, Pittalis, and Proetto correction to Becke & Johnson */
      {"XC_MGGA_X_2D_PRHG07", XC_MGGA_X_2D_PRHG07}, /* Pittalis, Rasanen, Helbig, Gross Exchange Functional */
      {"XC_MGGA_X_2D_PRHG07_PRP10", XC_MGGA_X_2D_PRHG07_PRP10}, /* PRGH07 with PRP10 correction */
      {"XC_MGGA_X_REVTPSS", XC_MGGA_X_REVTPSS}, /* revised Perdew, Tao, Staroverov & Scuseria exchange */
      {"XC_MGGA_X_PKZB", XC_MGGA_X_PKZB}, /* Perdew, Kurth, Zupan, and Blaha */
      {"XC_MGGA_X_M05", XC_MGGA_X_M05}, /* Worker for M05 functional */
      {"XC_MGGA_X_M05_2X", XC_MGGA_X_M05_2X}, /* Worker for M05-2X functional */
      {"XC_MGGA_X_M06_HF", XC_MGGA_X_M06_HF}, /* Worker for M06-HF functional */
      {"XC_MGGA_X_M06", XC_MGGA_X_M06}, /* Worker for M06 functional */
      {"XC_MGGA_X_M06_2X", XC_MGGA_X_M06_2X}, /* Worker for M06-2X functional */
      {"XC_MGGA_X_M08_HX", XC_MGGA_X_M08_HX}, /* Worker for M08-HX functional */
      {"XC_MGGA_X_M08_SO", XC_MGGA_X_M08_SO}, /* Worker for M08-SO functional */
      {"XC_MGGA_X_MS0", XC_MGGA_X_MS0}, /* MS exchange of Sun, Xiao, and Ruzsinszky */
      {"XC_MGGA_X_MS1", XC_MGGA_X_MS1}, /* MS1 exchange of Sun, et al */
      {"XC_MGGA_X_MS2", XC_MGGA_X_MS2}, /* MS2 exchange of Sun, et al */
      {"XC_MGGA_X_M11", XC_MGGA_X_M11}, /* Worker for M11 functional */
      {"XC_MGGA_X_M11_L", XC_MGGA_X_M11_L}, /* M11-L functional from Minnesota */
      {"XC_MGGA_X_MN12_L", XC_MGGA_X_MN12_L}, /* MN12-L functional from Minnesota */
      {"XC_MGGA_C_CC06", XC_MGGA_C_CC06}, /* Cancio and Chou 2006 */
      {"XC_MGGA_X_MK00", XC_MGGA_X_MK00}, /* Exchange for accurate virtual orbital energies */
      {"XC_MGGA_C_TPSS", XC_MGGA_C_TPSS}, /* Perdew, Tao, Staroverov & Scuseria correlation */
      {"XC_MGGA_C_VSXC", XC_MGGA_C_VSXC}, /* VSxc from Van Voorhis and Scuseria (correlation part) */
      {"XC_MGGA_C_M06_L", XC_MGGA_C_M06_L}, /* M06-Local functional from Minnesota */
      {"XC_MGGA_C_M06_HF", XC_MGGA_C_M06_HF}, /* Worker for M06-HF functional */
      {"XC_MGGA_C_M06", XC_MGGA_C_M06}, /* Worker for M06 functional */
      {"XC_MGGA_C_M06_2X", XC_MGGA_C_M06_2X}, /* Worker for M06-2X functional */
      {"XC_MGGA_C_M05", XC_MGGA_C_M05}, /* Worker for M05 functional */
      {"XC_MGGA_C_M05_2X", XC_MGGA_C_M05_2X}, /* Worker for M05-2X functional */
      {"XC_MGGA_C_PKZB", XC_MGGA_C_PKZB}, /* Perdew, Kurth, Zupan, and Blaha */
      {"XC_MGGA_C_BC95", XC_MGGA_C_BC95}, /* Becke correlation 95 */
      {"XC_MGGA_C_REVTPSS", XC_MGGA_C_REVTPSS}, /* revised TPSS correlation */
      {"XC_MGGA_XC_TPSSLYP1W", XC_MGGA_XC_TPSSLYP1W}, /* Functionals fitted for water */
      {"XC_MGGA_X_MK00B", XC_MGGA_X_MK00B}, /* Exchange for accurate virtual orbital energies (v. B) */
      {"XC_MGGA_X_BLOC", XC_MGGA_X_BLOC}, /* functional with balanced localization */
      {"XC_MGGA_X_MODTPSS", XC_MGGA_X_MODTPSS}, /* Modified Perdew, Tao, Staroverov & Scuseria exchange */
      {"XC_MGGA_C_TPSSLOC", XC_MGGA_C_TPSSLOC}, /* Semilocal dynamical correlation */
      {"XC_MGGA_X_MBEEF", XC_MGGA_X_MBEEF}, /* mBEEF exchange */
      {"XC_MGGA_X_MBEEFVDW", XC_MGGA_X_MBEEFVDW}, /* mBEEF-vdW exchange */
      {"XC_MGGA_XC_B97M_V", XC_MGGA_XC_B97M_V}, /* Mardirossian and Head-Gordon */
      {"XC_MGGA_X_MVS", XC_MGGA_X_MVS}, /* MVS exchange of Sun, Perdew, and Ruzsinszky */
      {"XC_MGGA_X_MN15_L", XC_MGGA_X_MN15_L}, /* MN15-L functional from Minnesota */
      {"XC_MGGA_C_MN15_L", XC_MGGA_C_MN15_L}, /* MN15-L functional from Minnesota */
      {"XC_MGGA_X_SCAN", XC_MGGA_X_SCAN}, /* SCAN exchange of Sun, Ruzsinszky, and Perdew */
      {"XC_MGGA_C_SCAN", XC_MGGA_C_SCAN}, /* SCAN correlation */
      {"XC_MGGA_C_MN15", XC_MGGA_C_MN15}, /* MN15 functional from Minnesota */
      {"XC_HYB_MGGA_X_DLDF", XC_HYB_MGGA_X_DLDF}, /* Dispersionless Density Functional */
      {"XC_HYB_MGGA_X_MS2H", XC_HYB_MGGA_X_MS2H}, /* MS2 hybrid exchange of Sun, et al */
      {"XC_HYB_MGGA_X_MN12_SX", XC_HYB_MGGA_X_MN12_SX}, /* MN12-SX hybrid functional from Minnesota */
      {"XC_HYB_MGGA_X_SCAN0", XC_HYB_MGGA_X_SCAN0}, /* SCAN hybrid */
      {"XC_HYB_MGGA_X_MN15", XC_HYB_MGGA_X_MN15}, /* MN15 functional from Minnesota */
      {"XC_HYB_MGGA_XC_M05", XC_HYB_MGGA_XC_M05}, /* M05 functional from Minnesota */
      {"XC_HYB_MGGA_XC_M05_2X", XC_HYB_MGGA_XC_M05_2X}, /* M05-2X functional from Minnesota */
      {"XC_HYB_MGGA_XC_B88B95", XC_HYB_MGGA_XC_B88B95}, /* Mixture of B88 with BC95 (B1B95) */
      {"XC_HYB_MGGA_XC_B86B95", XC_HYB_MGGA_XC_B86B95}, /* Mixture of B86 with BC95 */
      {"XC_HYB_MGGA_XC_PW86B95", XC_HYB_MGGA_XC_PW86B95}, /* Mixture of PW86 with BC95 */
      {"XC_HYB_MGGA_XC_BB1K", XC_HYB_MGGA_XC_BB1K}, /* Mixture of B88 with BC95 from Zhao and Truhlar */
      {"XC_HYB_MGGA_XC_M06_HF", XC_HYB_MGGA_XC_M06_HF}, /* M06-HF functional from Minnesota */
      {"XC_HYB_MGGA_XC_MPW1B95", XC_HYB_MGGA_XC_MPW1B95}, /* Mixture of mPW91 with BC95 from Zhao and Truhlar */
      {"XC_HYB_MGGA_XC_MPWB1K", XC_HYB_MGGA_XC_MPWB1K}, /* Mixture of mPW91 with BC95 for kinetics */
      {"XC_HYB_MGGA_XC_X1B95", XC_HYB_MGGA_XC_X1B95}, /* Mixture of X with BC95 */
      {"XC_HYB_MGGA_XC_XB1K", XC_HYB_MGGA_XC_XB1K}, /* Mixture of X with BC95 for kinetics */
      {"XC_HYB_MGGA_XC_M06", XC_HYB_MGGA_XC_M06}, /* M06 functional from Minnesota */
      {"XC_HYB_MGGA_XC_M06_2X", XC_HYB_MGGA_XC_M06_2X}, /* M06-2X functional from Minnesota */
      {"XC_HYB_MGGA_XC_PW6B95", XC_HYB_MGGA_XC_PW6B95}, /* Mixture of PW91 with BC95 from Zhao and Truhlar */
      {"XC_HYB_MGGA_XC_PWB6K", XC_HYB_MGGA_XC_PWB6K}, /* Mixture of PW91 with BC95 from Zhao and Truhlar for kinetics */
      {"XC_HYB_MGGA_XC_TPSSH", XC_HYB_MGGA_XC_TPSSH}, /* TPSS hybrid */
      {"XC_HYB_MGGA_XC_REVTPSSH", XC_HYB_MGGA_XC_REVTPSSH}, /* revTPSS hybrid */
      {"XC_HYB_MGGA_XC_M08_HX", XC_HYB_MGGA_XC_M08_HX}, /* M08-HX functional from Minnesota */
      {"XC_HYB_MGGA_XC_M08_SO", XC_HYB_MGGA_XC_M08_SO}, /* M08-SO functional from Minnesota */
      {"XC_HYB_MGGA_XC_M11", XC_HYB_MGGA_XC_M11}, /* M11    functional from Minnesota */
      {"XC_HYB_MGGA_X_MVSH", XC_HYB_MGGA_X_MVSH}, /* MVS hybrid */
      {"XC_HYB_MGGA_XC_WB97M_V", XC_HYB_MGGA_XC_WB97M_V} /* Mardirossian and Head-Gordon */
  };
  
  /// Interface class to Libxc.
  class XC_functional
  {
      private:
          
          std::string libxc_name_;
  
          int num_spins_;
          
          xc_func_type handler_;
  
          /* forbid copy constructor */
          XC_functional(const XC_functional& src) = delete;
  
          /* forbid assigment operator */
          XC_functional& operator=(const XC_functional& src) = delete;
  
          bool initialized_{false};
  
      public:
  
          XC_functional(const std::string libxc_name__, int num_spins__)
              : libxc_name_(libxc_name__),
                num_spins_(num_spins__)
          {
              /* check if functional name is in list */
              if (libxc_functionals.count(libxc_name_) == 0) {
                  std::stringstream s;
                  s << "XC functional " << libxc_name__ << " is unknown";
                  TERMINATE(s);
              }
  
              /* init xc functional handler */
              if (xc_func_init(&handler_, libxc_functionals.at(libxc_name_), num_spins_) != 0) {
                  TERMINATE("xc_func_init() failed");
              }
  
              initialized_ = true;
          }
  
          XC_functional(XC_functional&& src__)
          {
              this->libxc_name_  = src__.libxc_name_;
              this->num_spins_   = src__.num_spins_;
              this->handler_     = src__.handler_;
              this->initialized_ = true;
              src__.initialized_ = false;
          }
  
          ~XC_functional()
          {
              if (initialized_) {
                  xc_func_end(&handler_);
              }
          }
  
          const std::string name() const
          {
              return std::string(handler_.info->name);
          }
          
          const std::string refs() const
          {
              std::stringstream s;
              for (int i = 0; i < 5; i++) {
                  if (handler_.info->refs[i] == NULL) {
                      break;
                  }
                  s << std::string(handler_.info->refs[i]->ref);
                  if (strlen(handler_.info->refs[i]->doi) > 0) {
                      s << " (" << std::string(handler_.info->refs[i]->doi) << ")";
                  }
                  s << std::endl;
              }
              
              return s.str();
          }
  
          int family() const
          {
              return handler_.info->family;
          }
  
          bool is_lda() const
          {
              return family() == XC_FAMILY_LDA;
          }
  
          bool is_gga() const
          {
              return family() == XC_FAMILY_GGA;
          }
  
          int kind() const
          {
              return handler_.info->kind;
          }
  
          bool is_exchange() const
          {
              return kind() == XC_EXCHANGE;
          }
  
          bool is_correlation() const
          {
              return kind() == XC_CORRELATION;
          }
  
          bool is_exchange_correlation() const
          {
              return kind() == XC_EXCHANGE_CORRELATION;
          }
  
          void set_relativistic(bool enabled__)
          {
              if (is_exchange()) {
                  if (enabled__) {
                      xc_lda_x_set_params(&handler_, 4.0/3.0, XC_RELATIVISTIC, 0.0);
                  } else {
                      xc_lda_x_set_params(&handler_, 4.0/3.0, XC_NON_RELATIVISTIC, 0.0);
                  }
              }
          }
  
          /// Get LDA contribution.
          void get_lda(const int size, 
                       const double* rho, 
                       double* v, 
                       double* e)
          {
              if (family() != XC_FAMILY_LDA) {
                  TERMINATE("wrong XC");
              }
  
              /* check density */
              for (int i = 0; i < size; i++) {
                  if (rho[i] < 0) {
                      std::stringstream s;
                      s << "rho is negative : " << Utils::double_to_string(rho[i]);
                      TERMINATE(s);
                  }
              }
  
              xc_lda_exc_vxc(&handler_, size, rho, e, v);
          }
  
          /// Get LSDA contribution.
          void get_lda(const int size,
                       const double* rho_up,
                       const double* rho_dn,
                       double* v_up,
                       double* v_dn,
                       double* e)
          {
              if (family() != XC_FAMILY_LDA) {
                  TERMINATE("wrong XC");
              }
  
              std::vector<double> rho_ud(size * 2);
              /* check and rearrange density */
              for (int i = 0; i < size; i++) {
                  if (rho_up[i] < 0 || rho_dn[i] < 0) {
                      std::stringstream s;
                      s << "rho is negative : " << Utils::double_to_string(rho_up[i]) 
                        << " " << Utils::double_to_string(rho_dn[i]);
                      TERMINATE(s);
                  }
                  
                  rho_ud[2 * i]     = rho_up[i];
                  rho_ud[2 * i + 1] = rho_dn[i];
              }
              
              std::vector<double> v_ud(size * 2);
  
              xc_lda_exc_vxc(&handler_, size, &rho_ud[0], &e[0], &v_ud[0]);
              
              /* extract potential */
              for (int i = 0; i < size; i++) {
                  v_up[i] = v_ud[2 * i];
                  v_dn[i] = v_ud[2 * i + 1];
              }
          }
  
          void add_lda(const int size,
                       const double* rho_up,
                       const double* rho_dn,
                       double* v_up,
                       double* v_dn,
                       double* e)
          {
              if (family() != XC_FAMILY_LDA) {
                  TERMINATE("wrong XC");
              }
  
              std::vector<double> rho_ud(size * 2);
              /* check and rearrange density */
              for (int i = 0; i < size; i++) {
                  if (rho_up[i] < 0 || rho_dn[i] < 0) {
                      std::stringstream s;
                      s << "rho is negative : " << Utils::double_to_string(rho_up[i]) 
                        << " " << Utils::double_to_string(rho_dn[i]);
                      TERMINATE(s);
                  }
                  
                  rho_ud[2 * i]     = rho_up[i];
                  rho_ud[2 * i + 1] = rho_dn[i];
              }
              
              std::vector<double> v_ud(size * 2);
              std::vector<double> e_tmp(size);
  
              xc_lda_exc_vxc(&handler_, size, &rho_ud[0], &e_tmp[0], &v_ud[0]);
              
              /* extract potential */
              for (int i = 0; i < size; i++) {
                  v_up[i] += v_ud[2 * i];
                  v_dn[i] += v_ud[2 * i + 1];
                  e[i]    += e_tmp[i];
              }
          }
  
          /// Get GGA contribution.
          void get_gga(const int size,
                       const double* rho,
                       const double* sigma,
                       double* vrho,
                       double* vsigma,
                       double* e)
          {
              if (family() != XC_FAMILY_GGA) TERMINATE("wrong XC");
  
              /* check density */
              for (int i = 0; i < size; i++)
              {
                  if (rho[i] < 0.0)
                  {
                      std::stringstream s;
                      s << "rho is negative : " << Utils::double_to_string(rho[i]);
                      TERMINATE(s);
                  }
              }
  
              xc_gga_exc_vxc(&handler_, size, rho, sigma, e, vrho, vsigma);
          }
  
          /// Get spin-resolved GGA contribution.
          void get_gga(const int size, 
                       const double* rho_up, 
                       const double* rho_dn, 
                       const double* sigma_uu, 
                       const double* sigma_ud, 
                       const double* sigma_dd, 
                       double* vrho_up,
                       double* vrho_dn, 
                       double* vsigma_uu, 
                       double* vsigma_ud,
                       double* vsigma_dd,
                       double* e)
          {
              if (family() != XC_FAMILY_GGA) TERMINATE("wrong XC");
  
              std::vector<double> rho(2 * size);
              std::vector<double> sigma(3 * size);
              /* check and rearrange density */
              /* rearrange sigma as well */
              for (int i = 0; i < size; i++)
              {
                  if (rho_up[i] < 0 || rho_dn[i] < 0)
                  {
                      std::stringstream s;
                      s << "rho is negative : " << Utils::double_to_string(rho_up[i]) 
                        << " " << Utils::double_to_string(rho_dn[i]);
                      TERMINATE(s);
                  }
                  
                  rho[2 * i] = rho_up[i];
                  rho[2 * i + 1] = rho_dn[i];
  
                  sigma[3 * i] = sigma_uu[i];
                  sigma[3 * i + 1] = sigma_ud[i];
                  sigma[3 * i + 2] = sigma_dd[i];
              }
              
              std::vector<double> vrho(2 * size);
              std::vector<double> vsigma(3 * size);
  
              xc_gga_exc_vxc(&handler_, size, &rho[0], &sigma[0], e, &vrho[0], &vsigma[0]);
  
              /* extract vrho and vsigma */
              for (int i = 0; i < size; i++)
              {
                  vrho_up[i] = vrho[2 * i];
                  vrho_dn[i] = vrho[2 * i + 1];
  
                  vsigma_uu[i] = vsigma[3 * i];
                  vsigma_ud[i] = vsigma[3 * i + 1];
                  vsigma_dd[i] = vsigma[3 * i + 2];
              }
          }
  };
  
  }
  
  #endif // __XC_FUNCTIONAL_H__
  