Applications of linear response theories to compute the low-lying potential energy surfaces: state-specific MRCEPA-based approach

Linear response theories based on the state specific multi-reference coupled electron-pair approximation-like methods (SS-MRCEPA) (Chattopadhyay and Mahapatra 2004 J. Phys. Chem. A 108 11664) (MRCEPA-LRT) have been amply utilized for computing excited-state potential energy surfaces (PES). Our MRCEPA-LRT methods provide energies in a size-intensive manner. As a pilot application, the effectiveness of the method is demonstrated by computing the PES of some low-lying excited states including the ground state of the BeH2 molecule. This system is very effective and widely used to judge the potentiality of any state-specific theory since its ground state possesses degeneracy/quasi-degeneracy and also faces intruders at different regions of the PES