Hamiltonian and recontracted basis sets: ΔESR = EMP2-DK/cc-pVTZ-DK - EMP2/cc-pVTZ The last two terms are zero-point energy corrections scaled with a factor of 0.989 to account for deficiencies in the harmonic approximation and spin-orbit corrections considered only for atoms. The Correlation Consistent Composite Approach is available as a keyword in NWChem and GAMESS (ccCA-S4 and ccCA-CC(2,3)) Complete Basis Set methods (CBS) The Complete Basis Set (CBS) methods are a family of composite methods, the members of which are: CBS-4M, CBS-QB3, and CBS-APNO, in increasing order of accuracy. These methods offer errors of 2.5, 1.1, and 0.7 kcal/mol when tested against the G2 test set. The CBS methods were developed by George Petersson and coworkers, and they make extrapolate several single-point energies to the "exact" energy. In comparison, the Gaussian-n methods perform their approximation using additive corrections. Similar to the modified G2(+) method, CBS-QB3 has been modified by the inclusion of diffuse functions in the geometry optimization step to give CBS-QB3(+). The CBS family of methods is available via keywords in the Gaussian 09 suite of programs. Weizmann-n theories The Weizmann-n ab initio methods (Wn, n = 1–4) are highly accurate composite theories devoid of empirical parameters. These theories are capable of sub-kJ/mol accuracies in prediction of fundamental thermochemical quantities such as heats of formation and atomization energies, and unprecedented accuracies in prediction of spectroscopic constants. The Wn-P34 variants further extend the applicability from first- and second-row species to include heavy main-group systems (up to xenon). The ability of these theories to successfully reproduce the CCSD(T)/CBS (W1 and W2), CCSDT(Q)/CBS (W3), and CCSDTQ5/CBS (W4) energies relies on judicious combination of very large Gaussian basis sets with basis-set extrapolation techniques. Thus, the high accuracy of Wn theories comes with the price of a significant computational cost. In practice, for systems consisting of more than ~9 non-hydrogen atoms (with C1 symmetry), even the computationally more economical W1 theory becomes prohibitively expensive with current mainstream server hardware. In an attempt to extend the applicability of the Wn thermochemistry methods, explicitly correlated versions of these theories have been developed: Wn-F12 (n = 1–3) and more recently even a W4-F12 theory. W1-F12 was successfully applied to large hydrocarbons (e.g., dodecahedrane, as well as to systems of biological relevance (e.g., DNA bases). W4-F12 theory has been applied to systems as large as benzene. In a similar manner, the WnX protocols that have been developed independently further reduce