anchor.diff:          use excitations, only: excit
           use hamiltonian_hub_k, only: slater_condon2_hub_k_excit
           use fciqmc_data, only: tau
  -        use system, only: hub_k_coulomb, nalpha, nbeta, nvirt
  +        use system, only: hub_k_coulomb, nalpha, nbeta, nvirt_alpha
   
           type(det_info), intent(in) :: cdet
           type(excit), intent(out) :: connection
  
positive.code:         call choose_ij_hub_k(cdet%occ_list_alpha, cdet%occ_list_beta, connection%from_orb(1), connection%from_orb(2), ij_sym)
  
          ! 2. Chose a random pair of spin orbitals to excite to.
          call find_ab_hub_k(cdet%f, cdet%unocc_list_alpha, ij_sym, connection%to_orb(1), connection%to_orb(2), allowed_excitation)
          connection%nexcit = 2
  
          if (allowed_excitation) then
  
              ! 3. probability of this excitation.
              ! pgen = p(i,j) [ p(a|i,j) p(b|i,j,a) + p(b|i,j) p(a|i,j,b) ]
              ! pgen = 1/(nalpha*nbeta) [ 1/(nbasis-nel) + 1/(basis-nel) ]
              !                    2
              !      =  -------------------------
              !         nalpha*nbeta*(nbasis-nel)
              pgen = 2.0_dp/(nalpha*nbeta*nvirt)
  
              ! 4. |H_ij| is constant for this system.
              abs_hmatel = abs(hub_k_coulomb)
  
negative.code:         if (allowed_excitation) then
  
              ! 3. probability of this excitation.
              !    ***WARNING: dependent upon (arbitrary) implementation choices***
              !    See comments in gen_excit_init_hub_k_no_renorm.
              ! pgen = p(i,j) p(a|i,j) p(b|i,j,a)
              !      = 1/(nalpha*nbeta) 1/nvirt_alpha
              pgen = 1.0_p/(nalpha*nbeta*nvirt_alpha)
  
              ! 4. find the connecting matrix element.
              connection%nexcit = 2
  