Many-body perturbation and coupled-cluster methods using numerical Hartree-Fock and correlating orbitals are applied to diatomic molecules for the first time. For LiH, the correlation energy is within 0.001 a.u. of the nonrelativistic correlation energy limit, which provides an energy of -8.069 a.u. compared to the exact result of -8.070. These results suggest that highly accurate results for diatomic molecules may be obtained by the proposed approach.

• Received 27 September 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.426