Practice and Theory Two Sides of the Same Coin

Response functions are functions of the frequencies of the external fields, and although response theory is based on time-dependent perturbation theory, it is more appropriate to describe response theory as frequency-dependent rather than time-dependent. Nevertheless, response theory for approximate quantum chemical models is sometimes referred to as time-dependent, for example, time-dependent DFT (TDDFT) and time-dependent HF (TDHF) theory. Related by Fourier transformation, the time and frequency domains are two sides of the same coin. In practice, however, response functions are computed for a limited number of perturbation frequencies without any explicit reference to the time evolution of the quantum state. 

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