Accurate Open-Shell Noncovalent Interaction Energies from the Orbital-Optimized Moller-Plesset Perturbation Theory: Achieving CCSD Quality at the MP2 Level by Orbital Optimization

The accurate description of noncovalent interactions is one of the most challenging problems in modern computational chemistry, especially those for open-shell systems. In this study, an investigation of open-shell noncovalent interactions with the orbital-optimized MP2 and MP3 (OMP2 and OMP3) is presented. For the considered test set of 23 complexes, mean absolute errors in noncovalent interaction energies (with respect to CCSD(T) at complete basis set limits) are 0.68 (MP2), 0.37 (OMP2), 0.59 (MP3), 0.23 (OMP3), and 0.38 (CCSD) kcal mol(-1). Hence, with a greatly reduced computational cost, one may achieve CCSD quality at the MP2 level by orbital optimization [scaling formally as O(N-6) for CCSD compared to O(N-5) for OMP2, where N is the number of basis functions]. Further, one may obtain a considerably better performance than CCSD using the OMP3 method, which has also a lower cost than CCSD.