On the use of symmetry in first‐order perturbed Hartree–Fock theory

A method for the determination of the symmetry of first‐order vectors in Hartree–Fock perturbation theory is developed. This leads to the definition of symmetry‐adapted basis vectors to be employed at first order in the perturbation. It is shown that computer time can be saved, to some extent, in the calculation of second‐order properties, by exploiting molecular symmetry. Specific examples are given for methane, ammonia, and water.