Gaussian-2 (G2) theory: Reduced basis set requirements

Two variations of G2(MP2) theory which employ smaller basis sets in evaluating the quadratic configuration interaction QCISD(T) component of the energy are presented. The first, G2(MP2,SVP), uses the split-valence plus polarization (SVP) 6-31G(d) basis, while the second, G2(MP2,SV), uses the split-valence (SV) 6-31G basis. The methods are evaluated on the basis of results for the set of 125 systems used for testing G2 theory. The mean absolute deviation of G2(MP2,SVP) results from experimental values is 1.63 kcal mol−1 compared with 1.58 and 1.21 kcal mol−1 for G2(MP2) and G2, respectively. The G2(MP2,SVP) method thus provides results which are generally very similar in quality to those obtained from G2(MP2) but at considerably reduced computational expense. On the other hand, the mean absolute deviation of G2(MP2,SV) results from experiment is substantially larger (2.13 kcal mol−1). The G2(MP2,SV) method exceeds the 2 kcal mol−1 target accuracy of G2 theory for an unacceptably large number of comparisons.