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

The numerical gradient scheme for rigorously size-extensive spin-free state-specific multireference Møller–Plesset perturbation theory based on Rayleigh–Schrödinger 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 reflection of the ability of the present method in studying geometrical parameters of states plagued by electronic degeneracy.