Gaussian-2 theory for molecular energies of first- and second-row compounds

Abstract

The Gaussian-2 theoretical procedure (G2 theory), based on ab initio molecular orbital theory, for calculation of molecular energies (atomization energies, ionization potentials, electron affinities, and proton affinities) of compounds containing first- (Li–F) and second-row atoms (Na–Cl) is presented. This new theoretical procedure adds three features to G1 theory [J. Chem. Phys. 90, 5622 (1989)] including a correction for nonadditivity of diffuse-sp and 2df basis set extensions, a basis set extension containing a third d function on nonhydrogen and a second p function on hydrogen atoms, and a modification of the higher level correction. G2 theory is a significant improvement over G1 theory because it eliminates a number of deficiencies present in G1 theory. Of particular importance is the improvement in atomization energies of ionic molecules such as LiF and hydrides such as C2H6, NH3, N2H4, H2O2, and CH3SH. The average absolute deviation from experiment of atomization energies of 39 first-row compounds is reduced from 1.42 to 0.92 kcal/mol. In addition, G2 theory gives improved performance for hypervalent species and electron affinities of second-row species (the average deviation from experiment of electron affinities of second-row species is reduced from 1.94 to 1.08 kcal/mol). Finally, G2 atomization energies for another 43 molecules, not previously studied with G1 theory, many of which have uncertain experimental data, are presented and differences with experiment are assessed.

Nonempirical Double-Hybrid Functionals: An Effective Tool for Chemists

“…Figure 1 shows the impact of the exact-exchange fraction on the accuracy for the atomization energy of the G2-148 dataset. 41 Indeed, the ¼ factor of PBE0, is very close to the minimum. Very recently, another dependence of the integrand on λ was proposed, leading to a different integer n (such as n=3).…”

“…From Figure 1, which illustrates the influence of the relative amount of the exact-exchange and of the PT2 correlation contribution on the performances of the given DH on the G2 atomization dataset, 41 it can be noticed how non-empirical functionals perform very well, all lying close to the belt defined by lowest deviations. Subsequent works introduced system dependency (PBE-ACDH) 46 or fitting procedure (PBE-QACF-2) 47 and provide similar errors on standard databases of reactions.…”

Ab Initio Potential Energy Surface for H + OCS Reactions: Extended Basis Sets and Correlation Treatment

“…It might be worth pointing out that the 6-31 1+G(2df,2p) basis set is larger in both the valence and polarization AO spaces than the 6-31G * basis used in Pople et al's GI and G2 approaches (Pople ct al. 1989;Curtiss et al 1991). Finally, to check what effect correlation has on the optimized structures, the geometries for these five points were reoptiminzed using QCISD/6-31 l+G(2df,2p), followed by an energy refinement at QCISD(T)/6-31 l+G(2df,2p).…”

Structures and stabilities of CaC3 isomers

“…The G1 and G2 [26] methods have been applied in order to compute accurate relative energies. Since spin-contamination might affect the reliability of these methods, approximate projected MP values were used to compute electronic energies.…”