Improved third-order Møller-Plesset perturbation theory.

Based on a partitioning of the total correlation energy into contributions from parallel- and antiparallel-spin pairs of electrons, a modified third-order Møller-Plesset (MP) perturbation theory is developed. The method, termed SCS-MP3 (SCS for spin-component-scaled) continues previous work on an improved version of MP2. A benchmark set of 32 isogyric reaction energies, 11 atomization energies, and 11 stretched geometries is used to assess to performance of the model in comparison to the standard quantum chemical approaches MP2, MP3, and QCISD(T). It is found, that the new method performs significantly better than usual MP2/MP3 and even outperforms the more costly QCISD method. Opposite to the usual MP series, the SCS third-order correction uniformly improves the results. Dramatic enhancements are especially observed for the more difficult atomization energies, some of the stretched geometries, and reaction and ionization energies involving transition metal compounds where the method seems to be competitive or even superior to the widely used density functional approaches. Further tests performed for other complex systems (biradicals, C(20) isomers, transition states) demonstrate that the SCS-MP3 model yields often results of QCISD(T) accuracy. The uniformity with which the new approach improves for very different correlation problems indicates significant robustness, and suggests it as a valuable quantum chemical method of general use.