Configuration interaction: Molecular orbitals for accurate calculations on diatomics

A procedure is developed to construct an optimum set of molecular orbitals (MO's) to be used in large scale configuration interaction expansion for diatomics. The set is optimum in the sense that a significant energy improvement can be obtained for a relatively short wavefunction expansion. Application of this methodology to the ground state of the LiH molecule gave an energy of –8.06345 a.u. for an expansion with 1852 terms obtained from a set with 16-, 12-, and 6-type MO's.