Extrapolation in configuration interaction calculations using perturbation theory natural orbitals: Applications to H2O and HeH2+

A procedure for carrying out the Buenker--Peyerimhoff extrapolation using perturbation theory natural orbitals (PTNO's) is presented. This procedure is then applied to the H2O and HeH2+ systems within the framework of a configuration space comprising all single and double excitations from a single configuration. The results indicate that significantly smaller secular determinants need to be solved when PTNO's are used; the extrapolated CI energies are within 5 x 10-4 a.u. of energy obtained from the full calculation within the same configuration space