Time-dependent density functional theory within the Tamm–Dancoff approximation

A computationally simple method for molecular excited states, namely, the Tamm–Dancoff approximation to time-dependent density functional theory, is proposed and implemented. This method yields excitation energies for several closed- and open-shell molecules that are essentially of the same quality as those obtained from time-dependent density functional theory itself, when the same exchange-correlation functional is used.