Many-body perturbation calculation on Be using a multiconfiguration model space and an intermediate Hamiltonian

We report a perturbation calculation of the ground state energy of the Be atom starting from a multiconfigurational model space and using the new concept of an intermediate Hamiltonian. The traditional MC-MBPT is known to diverge for this problem, whereas our method strongly converges. This convergence is explained by the fact that this method is free from any intruder state problem. The second-order calculation yields 140% of the correlation energy with a single reference model space, 93% with a multireference, and 98% with our method demonstrating the efficiency of the intermediate Hamiltonian.