title: Improved virtual orbitals in state specific multireference perturbation theory for prototypes of quasidegenerate electronic structure.
abstract: The state-specific multireference perturbation theory (SSMRPT) with an improved virtual orbital complete active space configuration interaction (IVO-CASCI) reference function [called as IVO-SSMRPT] is used to investigate the energy surface, geometrical parameters, molecular properties of spectroscopic interest for the systems/situations [such as BeH2, BeCH2, MgCH2, Si2H4, unimolecular dissociation of H2CO, and intramolecular reaction pathways of 1,3-butadiene] where the effect of quasidegeneracy cannot be neglected. The merit of using the IVO-CASCI rather than complete active space self-consistent field (CASSCF) is that it is free from iterations beyond those in the initial SCF calculation and the convergence difficulties that plague CASSCF calculations with increasing size of the CAS. While IVO-CASCI describes the non-dynamical correlation, the SSMRPT scheme is a good second-order perturbative approximation to account for the rest of the correlation energy. Our IVO-SSMRPT method is instrumental in avoiding intruder states in an size-extensive manner and allows the revision of the content of wave function in the model space. It can treat model as well as real systems with predictive accuracy, as is evident from the fairly nice accordance between our estimates, and high-level theoretical results. Our estimates also corroborate well with some experimental findings.
authors.affiliation: Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, India.
authors.forename: Suvonil
authors.initials: S
authors.lastname: Sinha Ray
authors.affiliation: Department of Chemistry, Jhargram Raj College, Jhargram 721507, India.
authors.forename: Pradipta
authors.initials: P
authors.lastname: Ghosh
authors.affiliation: Indian Institute of Astrophysics, Bangalore 560034, India.
authors.forename: Rajat K
authors.initials: RK
authors.lastname: Chaudhuri
authors.affiliation: Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, India.
authors.forename: Sudip
authors.identifier: 0000000234776013
authors.initials: S
authors.lastname: Chattopadhyay