Study of equilibrium geometries of diradicaloid systems via state specific multireference Moller-Plesset perturbation theory (SS-MRMPPT)

Abstract

The numerical gradient scheme for rigorously size-extensive spin-free state-speci. c multireference Moller-Plesset perturbation theory based on Rayleigh-Schrdinger expansion (termed as SS-MRMPPT) has been employed for calculating equilibrium geometrical parameters of diradical systems such as singlet CH2, m-benzyne and 2,6-pyridyne. These systems possess quasi-degeneracy of varying degrees. A close and quantitative agreement of results obtained by low cost SS-MRMPPT method with the results of highly sophisticated ab initio methods is an encouraging re. ection of the ability of the present method in studying geometrical parameters of states plagued by electronic degeneracy.