The binding energy of the ground state of Be2

Abstract

Two convergent sequences of multiconfiguration self‐consistent field and configuration interaction (CI) calculations have confirmed the predictions of a previous interacting correlated fragments (ICF) calculation and given best values for the D_e and R_e of Be₂(X ¹Σ⁺) of 2.04±0.21 kcal/mol and 4.73±0.03a₀, respectively. These calculations used a 6s/4p/3d/1f Slater basis set on each atom and correlated only the four 2s electrons. They were shown computationally to be free from basis set superposition and size‐consistency errors. A large second order CI calculation yielded a rigorous lower bound to the full four‐electron CI binding energy of 1.87 kcal/mol within the present basis set. Arguments are presented that the 1s electronic correlation effect on the binding energy is less than 0.2 kcal/mol. The earlier ICF calculation on Be₂ has been repeated and extended to a higher level of fragment correlation using the current basis set. The best ICF results are in close agreement with the second order CI calculations, indicating that the full four‐electron CI limit within this basis set has been reached and providing the first computational proof of the convergence of ICF method.