State specific multireference moller–plesset perturbation theory: a few applications to ground, excited and ionized states

Abstract

We provide further tests and illustrations of the complete active space based statespecificmultireferenceMøller–Plessetperturbationtheory (SS-MRMPPT) which opens the way for the treatment of dynamic correlations in situations containing significant static correlations also in an accurate, size- extensive and intruder free manner enjoying at the same time a very favorable computational cost. We have investigated various interesting and computationally challenging systems [H2O, H2O+, BeC, MgC, CO+, Be3, benzene, trimethylenemethane and 1,2,3-tridehydrobenzene] in either their ground, or excited/ionizedstates. It is found that SS-MRMPPT calculations provide very encouraging results which can be meaningfully compared with other state-of-the-art theoretical estimates. Present results convincingly indicate that the SS-MRMPPT method is not only successful in portraying situations that warrant MR description but also performs acceptably good in cases where a naive single-reference method is enough which reinforces the claim that the SS-MRMPPT is a very useful and flexible ab initio method.