Efficient algorithm for calculating noninteracting frequency-dependent linear response functions

A simple algorithm for calculating frequency-dependent noninteracting linear response functions is reported. The algorithm decouples the frequency dependence from the state dependence so that the computation is essentially reduced to that of the static response function. The computational time becomes almost independent of the number of frequencies and the accuracy can be systematically improved. Using the present scheme, approximations based on the static response function, such as the plasmon-pole approximation, may be avoided.