Spin-component scaled second-order Møller–Plesset perturbation theory for the calculation of molecular geometries and harmonic vibrational frequencies

The implementation and performance of an analytic gradient with respect to nuclear displacements for the spin-component scaled MP2 method (SCS-MP2) is presented. The geometries and vibrational frequencies of a benchmark set of 31 first-row molecules are considered. The new SCS-MP2 method performs significantly better than standard MP2: the RMS deviation (mean absolute error) for bond lengths drops from 1.2 (0.8) to 0.9 (0.6) pm and from 84 (67) to 57 (48) cm−1 for the vibrational frequencies. As further tests the geometries of weakly bonded systems, transition metal compounds and organic transition states are considered. For almost all systems the SCS-MP2 method outperforms the MP2 approach albeit for the same computational cost. Significant improvements are especially observed for cases which are usually known as MP2 pitfalls.