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Phys. Chem. Chem. Phys., 2009, 11, 4728 - 4741, DOI: 10.1039/b822910d

A companion perturbation theory for state-specific multireference coupled cluster methods

Francesco A. Evangelista, Andrew C. Simmonett, Henry F. Schaefer III, Debashis Mukherjee and Wesley D. Allen

A partitioning scheme is applied to the state-specific Mukherjee multireference coupled cluster method to derive a companion perturbation theory (Mk-MRPT2). A production-level implementation of Mk-MRPT2 is reported. The effectiveness of the Mk-MRPT2 method is demonstrated by application to the classic F2 dissociation problem and the lowest-lying electronic states of meta-benzyne, including computations with up to 766 atomic orbitals. We show that Mk-MRPT2 theory is particularly useful in multireference focal point extrapolations to determine ab initio limits.

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