A simplified ab initio treatment of diradicaloid structures produced from stretching and breaking chemical bonds.

The present investigation reports on the prospect of using state specific multireference perturbation theory (SSMRPT) with an improved virtual orbital complete active space configuration interaction (IVO-CASCI) reference function (IVO-SSMRPT) to generate potential energy surfaces (PESs) for molecular systems [such as CH, CH, CH, HO, LiH, and KN] by stretching and breaking of suitable bonds with modest basis sets. We have also revisited the dissociation energy profile of triplet ketene which exhibits a step-like structure in the observed rate. The application of the method has also been made to the ionization energies of HO.

Improved virtual orbitals in state specific multireference perturbation theory for prototypes of quasidegenerate electronic structure.

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 BeH, BeCH, MgCH, SiH, unimolecular dissociation of HCO, 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.

Four-Component Relativistic State-Specific Multireference Perturbation Theory with a Simplified Treatment of Static Correlation.

The relativistic multireference (MR) perturbative approach is one of the most successful tools for the description of computationally demanding molecular systems of heavy elements. We present here the ground state dissociation energy surfaces, equilibrium bond lengths, harmonic frequencies, and dissociation energies of Ag, Cu, Au, and I computed using the four-component (4c) relativistic spinors based state-specific MR perturbation theory (SSMRPT) with improved virtual orbital complete active space configuration interaction (IVO-CASCI) functions. The IVO-CASCI method is a simple, robust, useful and lower cost alternative to the complete active space self-consistent field approach for treating quasidegenerate situations.

Communication: Viewing the ground and excited electronic structures of platinum and its ion through the window of relativistic coupled cluster method.

Highly accurate electronic structure calculations are often needed to supplement scant experimental data. We report the ground D and some selected low lying excited/ionized states of Pt and its ions obtained using the Fock space multireference coupled cluster method with four-component relativistic spinors. The present work establishes the stability of the S state of its negative ion and reproduces the binding energy of this state within 10 cm.

Taming the Electronic Structure of Diradicals through the Window of Computationally Cost Effective Multireference Perturbation Theory.

Recently a state-specific multireference perturbation theory (SSMRPT) with an improved virtual orbitals complete active space configuration interaction (IVO-CASCI) reference function has been proposed for treating electronic structures of radicals such as methylene, m-benzyne, pyridyne, and pyridynium cation. This new development in MRPT, termed as IVO-SSMRPT, ensures that it is able to describe the structure of radicaloids with reasonable accuracy even with small reference spaces. IVO-SSMRPT is also capable of predicting the correct ordering of the lowest singlet-triplet gaps.