Abstract:
Adaptation of improved virtual orbital complete active space configuration interaction functions in state-specific multireference perturbation
theory motivated by the Brillouin-Wigner perturbation scheme using Moller-Plesset multipartitioning is examined. The method, denoted
as IVO-BWMRPT, focuses on only the root of principal interest at a time using single-root parameterization of Jeziorski-Monkhorst ansatz
within the frame of an effective Hamiltonian. This approach yields size-extensive energy and avoids intruder-state problems in a natural
manner. It allows relaxation of the reference space wave function in the presence of the perturbation which produces an important differential
effect on the energy and cannot be neglected for quasidegenerate electronic states. The method has been tested against nontrivial
situations such as the Be + H2 insertion profile along with the energy surfaces of FH and X2 (X = F, Cl, and Br), in which conventional
single-reference methods generally fail, exhibiting very encouraging findings. We also consider the energy surfaces of ethylene (by breaking
the π bond as well as the CC bond) and for the twisting of tetramethyleneethane. IVO-BWMRPT represents a rather balanced protocol for the
description of molecules at a wide range of geometries, including stretched or dissociating bonds. Close agreement of our estimates with the
reference values provides a useful measure for the success of the IVO-BWMRPT method to treat strongly correlated systems. Our results for
TME show that the singlet state always lies below the triplet state for different conformations. The IVO-BWMRPT furnishes a compact and
correct representation of the MR-wave function, and hence, a large variety of quasidegenerate situations can be accommodated within the
method.
