A theoretical study of $BeN$ linear chains: Variational and perturbative approaches

A systematic theoretical study of $BeN$ linear chains by means of both variational (multireference configuration interaction) and perturbative (⁠$n$-electron valence state perturbation theory) methodologies is reported. Extensive calculations have been performed using atomic natural orbital basis set of increasing size (⁠$3s1p$⁠, $4s2p1d$⁠, and $5s3p2d1f$⁠). The problematic task of obtaining a coherent description of the potential energy surface with approximate ab initio methods is addressed. The main difficulty met in the computation of the binding energy of long $BeN$ chains essentially arises from the need of using, as the dissociation is approached, a variational space different from that suitable to treat the system near the equilibrium distance. To overcome this problem, two alternative approaches are presented and the computed dissociation energies are compared to the corresponding full-CI values reported in a recent work [V. Vetere et al., J. Chem. Phys. 130, 024301 (2009)].