The Random Phase Approximation (RPA),111, 341 also known as time-dependent Hartree-Fock (TD-HF) theory, is an alternative to CIS for uncorrelated calculations of excited states. It offers some advantages for computing oscillator strengths, e.g., exact satisfaction of the Thomas-Reike-Kuhn sum rule,630 and is roughly comparable in accuracy to CIS for singlet excitation energies, but is inferior for triplet states. RPA energies are non-variational, and in moving around on excited-state potential energy surfaces, this method can occasionally encounter singularities that prevent numerical solution of the underlying equations,190 whereas such singularities are mathematically impossible in CIS calculations.