Gaussian-3 Theory Using Density Functional Geometries and Zero-Point Energies

In a previous paper, we described a general procedure referred to as Gaussian-3 (G3) theory for the quantum chemical computation of total energies of molecules at their equilibrium geometries. The G3 method has been assessed on a total of 299 energies (enthalpies of formation, ionization energies, electron affinities, and proton affinities) where accurate experimental information is available. The average absolute deviation from experiment of G3 theory for these energies is 1.01 kcal/mol. In G3 theory, the geometries are obtained from second-order Moller-Plesset perturbation theory (MP2) while the zero-point energies are obtained from scaled Hartree-Fock (HF) frequencies. There are several cases where improved vibrational frequencies including electron correlation effects are necessary to obtain accurate zero-point energies. In this work, a variation of Gaussian-3 (G3) theory is presented in which the geometries and zero-point energies are obtained from B3LYP density functional theory. This variation, referred to as G3/B3LYP, is assessed on the same test set of molecules. The average absolute deviation from experiment of G3/B3LYP theory for the 299 energies falls slightly to 0.99 kcal/mol. Significant improvement is obtained for a few systems involving multiple bonding due to the use of better geometries and zero-point energies. Somewhat poorer performance is seen for some ionization potentials in systems involving Jahn-Teller distortion in the cations. Overall, G3/B3LYP appears to be a useful alternative to conventional G3 theory.