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 [ B3LYP/6-31G(d)] instead of geometries from second-order perturbation theory [MP2(FU)/6-31G(d)] and zero-point energies from Hartree-Fock theory [HF/6- 31G(d)]. This variation, referred to as G3//B3LYP, is assessed on 299 energ ies (enthalpies of formation, ionization potentials, electron affinities, p roton affinities) from the G2/97 test set [J. Chem. Phys. 109, 42 (1998)]. The G3//B3LYP average absolute deviation from experiment for the 299 energi es 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. G3//B3LYP theory gives significantly improved results for several cases for which MP2 theory is deficient for optimized geometries, such as CN and O-2(+). Howev er, G3//B3LYP does poorly for ionization potentials that involve a Jahn-Tel ler distortion in the cation (CH4+, BF3+, BCl3+) because of the B3LYP/6-31G (d) geometries. The G3(MP2) method is also modified to use B3LYP/6-31G(d) g eometries and zero-point energies. This variation, referred to as 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. (C) 1999 American Institute of P hysics. [S0021-9606(99)30512-2].