On the calculation of natural orbitals by perturbation theory

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Abstract
Natural orbitals are calculated by the application of perturbation theory without the use of many‐body techniques. They are obtained by diagonalization of a one‐electron density matrix, the elements of which are approximated by perturbation theory using the Bk method. Calculations on the H2O and O2 molecules demonstrate that these approximate NO's do not differ significantly from the exact NO's and that these NO's may be used to reduce the amount of effort in a configuration interaction calculation. Comparisons are also made with the iterative natural orbital approach.