Further development of SDSPT2 for strongly correlated electrons

The static-dynamic-static multi-state multi-reference second-order perturbation theory (SDS-MSMRPT2 or SDSPT2 in short), which employs a multi-partitioned (state-dependent) Moller-Plesset-like diagonal operator for the zeroth-order Hamiltonian and individual configuration state functions for the perturbers, is reconsidered by making use of the state-universal Dyall Hamiltonian for the zeroth-order Hamiltonian and the union of spin-adapted, internally contracted basis functions generated from all the reference functions for the perturbers. The graphical unitary group approach along with particle-hole correspondence is employed in the evaluation of Hamiltonian matrix elements. This particular variant of SDSPT2 can also be regarded as an extension of the spin-adapted, partially contracted variant of NEVPT2 (n-electron valence state second-order perturbation theory). The performance of SDSPT2 is examined by taking C-2, LiF, and Fe(C5H5)(2) as examples. The results show that the low-lying states of the systems can be obtained very accurately.