Generation of potential energy curves for the X-1Sigma (+) g, B-1 Delta (+) (g), and B' (1) states of C-2 using the effective valence shell Hamiltonian method

Abstract

Calculations of the ground and excited state potential energy curves of C2 using the third-order effective valence Hamiltonian (H/Sup-3rd(v) ) method are benchmarked against full configuration interaction and other correlated single-reference perturbative and nonperturbative theories. The large nonparallelity errors (NPEs) exhibited even by state-of-art coupled cluster calculations through perturbative triples indicate a serious deficiency of these single-reference theories. The Hv method, on the other hand, produces a much reduced NPE, rendering it a viable approximate many-body method for accurately determining global ground and excited state potential energy curves/surfaces.