Singular points of frequency-dependent polarizabilities from time-dependent Hartree--Fock theory

It is shown that, with a simple basis set, the singular frequencies calculated from time-dependent Hartree--Fock (TDHF) theory can be equal to those calculated from the virtual orbital approximation to electronic excitation energies. The well known inadequacies of this approximation suggest that TDHF theory may be of limited value for the study of atomic and molecular excited states, unless special care is exercised in the selection of the atomic orbital basis set