The binding energy of the ground state of Be₂

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 De and Re of Be2(X 1∑+) of 2.04±0.21 kcal/mol and 4.73±0.03a0, respectively. These calculations used a 6s/4/7/3d/1/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. © 1983 American Institute of Physics.