Second-order Møller-Plesset theory is one of the most basic wave function approaches which includes electron correlation directly. Due to its simplicity, the MP2 method is often the best level one can afford for a larger molecular system. At the other end of the spectrum, the MP2-R12 method of Kutzelnigg, Klopper, and co-workers is a promising approach to computing MP2 energies in the complete basis set limit for smaller systems. PSI3 is one of the very few publicly available programs to feature a robust implementation of the MP2-R12 method.
PSI3 is capable of computing closed-shell MP2 and MP2-R12/A energies using integral-direct techniques and a multithreaded algorithm, which lends itself perfectly for execution on symmetric multiprocessor (SMP) machines. PSI3 is also capable of computing RHF, UHF, and ROHF (using semicanonical orbitals) MP2 energies and one-particle density matrices, and RHF MP2 analytic gradients. Occupied and virtual orbitals can be frozen during the energy calculation, but not for the calculation of the one-particle density matrix or the analytic gradient.
Table 7 summarizes these capabilities.
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