Further development of SDSPT2 for strongly correlated electrons

The static-dynamic-static multi-state multi-reference second-order perturbation theory (SDS-MS-MRPT2 or SDSPT2 in short), which employs a multi-partitioned (state-dependent) Møller–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 C₂, LiF, and Fe(C₅H₅)₂ as examples. The results show that the low-lying states of the systems can be obtained very accurately.