A Single Reference Perturbation Theory beyond the Møller-Plesset Partition.

A new single reference perturbation partition is proposed for restricted open-shell Hartree-Fock (ROHF) and complete active space self-consistent field (CASSCF) orbitals. It is a sum of one-particle operator which is implicitly defined. When the operator acts on a ROHF or CASSCF orbital, the resultant eigenvalue is the orbital's corresponding orbital energy coming from the ROHF or CASSCF calculation. HF, F2, and N2 with stretched bonds are used for the size extensivity test. Results indicate that the first three-order energies calculated with the new partition are size extensive. Single reference perturbation calculations for H2O, NH3, and CH4 with CASSCF orbitals have been performed and compared with other methods like MRCI, MRCI+Q, MRPT2, and MRPT3. The single reference nature of the present perturbation theory is also shown with computations of the singlet-triplet separation of the CH2 and SiH2 radicals.