Internally contracted multi-reference second order perturbation theory based on hole-particle symmetry

Abstract

We present an internally contracted multi-reference second order perturbation theory employing the graphical unitary group approach (GUGA) and hole-particle symmetry. It is very convenient to realize internal contraction of any configuration space, reducing the performance of computing perturbation matrix elements. The preliminary tests show that our method completely avoids intruder state and deals with state degenerate problem. The spin-state splittings of ferrocene and energy difference between two isomers of di-copper complexes of prove that it is relatively accurate for our method to calculate medium molecular systems.