Gaussian-3 theory using density functional geometries and zero-point energies

A variation of Gaussian-3 (G3) theory is presented in which the geometries and zero-point energies are obtained from B3LYP density functional theory $[\mathrm{B3LYP/6-31G}\mathrm{(d)]}$ instead of geometries from second-order perturbation theory $[\mathrm{MP2(FU)/6-31G}\mathrm{(d)]}$ and zero-point energies from Hartree–Fock theory $[\mathrm{HF/6-31G}\mathrm{(d)].}$ This variation, referred to as $\mathrm{G3//B3LYP},$ is assessed on 299 energies (enthalpies of formation, ionization potentials, electron affinities, proton affinities) from the G2/97 test set [J. Chem. Phys. 109, 42 (1998)]. The $\mathrm{G3//B3LYP}$ average absolute deviation from experiment for the 299 energies is 0.99 kcal/mol compared to 1.01 kcal/mol for G3 theory. Generally, the results from the two methods are similar, with some exceptions. $\mathrm{G3//B3LYP}$ theory gives significantly improved results for several cases for which MP2 theory is deficient for optimized geometries, such as CN and ${\mathrm{O}}_{\mathrm{2}}^{\mathrm{+}}.$ However, $\mathrm{G3//B3LYP}$ does poorly for ionization potentials that involve a Jahn–Teller distortion in the cation $({\mathrm{CH}}_{\mathrm{4}}^{\mathrm{+}}\mathrm{,}$ ${\mathrm{BF}}_{\mathrm{3}}^{\mathrm{+}}\mathrm{,}$ ${\mathrm{BCl}}_{\mathrm{3}}^{\mathrm{+}}\mathrm{)}$ because of the $\mathrm{B3LYP/6-31G}\mathrm{(d)}$ geometries. The G3(MP2) method is also modified to use $\mathrm{B3LYP/6-31G}\mathrm{(d)}$ geometries and zero-point energies. This variation, referred to as $\mathrm{G3(MP2)//B3LYP},$ has an average absolute deviation of 1.25 kcal/mol compared to 1.30 kcal/mol for G3(MP2) theory. Thus, use of density functional geometries and zero-point energies in G3 and G3(MP2) theories is a useful alternative to MP2 geometries and HF zero-point energies.