Quantum chemistry composite methods

The first systematic model chemistry of this type with broad applicability was called Gaussian-1 (G1) introduced by John Pople.

The G2(+) variant, where the "+" symbol refers to added diffuse functions, better describes anions than conventional G2 theory.

G4 is a compound method in spirit of the other Gaussian theories and attempts to take the accuracy achieved with G3X one small step further.

Thus, Gaussian 4 (G4) theory [4] is an approach for the calculation of energies of molecular species containing first-row, second-row, and third row main group elements.

[5] A variant of G4MP2, termed G4(MP2)-6X, has been developed with an aim to improve the accuracy with essentially identical quantum chemistry components.

Unlike fixed-recipe, "model chemistries", the FPD approach[9][10][11][12][13] consists of a flexible sequence of (up to) 13 components that vary with the nature of the chemical system under study and the desired accuracy in the final results.

As with some other approaches, additive corrections for core/valence, scalar relativistic and higher order correlation effects are usually included.

[1] is an efficient computational approach developed for calculating accurate heats of formation of uncharged, closed-shell molecules comprising H, C, N, O, F, Si, P, S, Cl and Br, within experimental error.

Atom counts, Mulliken bond orders and HF/6-31G* and RI-MP2 energies are introduced as variables in a linear regression fit to a set of 1126 G3(MP2) heats of formation.

T1 reproduces experimental heats of formation for a set of 1805 diverse organic molecules from the NIST thermochemical database[14] with mean absolute and RMS errors of 8.5 and 11.5 kJ/mol, respectively.

The B3LYP density functional method with the cc-pVTZ basis set, and cc-pV(T+d)Z for third row elements (Na - Ar), are used to determine the equilibrium geometry.

[22] The Weizmann-n ab initio methods (Wn, n = 1–4)[23][24][25] are highly accurate composite theories devoid of empirical parameters.

[27] The Wn-P34 variants further extend the applicability from first- and second-row species to include heavy main-group systems (up to xenon).

[28] 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.

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.

[30] W1-F12 was successfully applied to large hydrocarbons (e.g., dodecahedrane,[31] as well as to systems of biological relevance (e.g., DNA bases).