Correlation energies of Be-like atoms: A multimodel space many-body perturbation calculation with finite basis sets

With finite basis sets constructed from B splines, a systematical many-body perturbation calculation of the Be-like atoms with Z from 3 to 36 is performed. A two-configuration model space (2s+2p)² is adopted in the calculation in order to speed the perturbation convergence. With perturbations up to the second order, we obtain 93--99.8 % of the total correlation energies given by Davidson et al. [Phys. Rev. A 44, 7071 (1991)] for Z from 4 to 20. The discrepancies between experimental and theoretical values approach a constant of 0.0005 while atomic number Z increases.