Study of the Ground State Dissociation of Diatomic Molecular Systems Using State-Specific Multireference Perturbation Theory: A Brillouin-Wigner Scheme. 
The size-extensive second-order state-specific (or single root) multireference (MR) perturbation theory (SS-MRPT) in the Brillouin-Wigner (BW) form using Mϕller-Plesset perturbative evaluations of orders up to 2 [termed as SS-MRMPPT(BW)] presents a viable, as well as promising, approach to include both nondynamic and dynamic correlations in the study of the bond-stretching (in multireference/quasidegenerate situations) of molecular species with a manageable cost/accuracy ratio.  It combines numerical stability in the presence of an intruder state problem with strict size consistency (when localized orbitals are used).  In this paper, the SS-MRMPPT(BW) method has been shown to properly break the bonds (in the ground state) of several diatomic molecules (such as F2, Cl2 and Br2, and BH) that have posed a severe challenge to any many-body theoretical approach due to the presence of quasidegeneracy of varying degrees in the ground state.  A comparison of the resulting potentials with the various theoretical results reveals that the method represents a valuable tool that is capable of properly accounting even for very strong quasidegeneracies, while also performing well in nondegenerate situations.  In this work, we have also calculated spectroscopic constants (such as equilibrium bond lengths, vibrational frequencies, and dissociation energies) of the ground state of these molecular systems.  The SS-MRMPPT spectroscopic constants are compared with the most accurate available ab initio calculations and other theoretical estimates of previous works to calibrate the efficacy of the method.  For the sake of completeness, we also compare the computed spectroscopic constants with the experimental observations.  The accuracy of computed spectroscopic parameters appears to be rather consistent over a multitude of systems for various basis sets.  The SS-MRMPPT enables quantitatively accurate and computationally affordable analysis of potential energy surfaces and spectroscopic constants of various multireference systems in the ground state.  It is particularly visible for spectroscopic parameters and nonparallelism error (NPE) calculations.  The calculations further reveal that the SS-MRMPPT(BW) method compared to its Rayleigh-Schrödinger counterpart [SS-MRMPPT(RS)] provides a more accurate and consistent solution for the whole dissociation path and spectroscopic constants.