(4.4.7) Romaniello : An elegant but not often used (I do not know why) method for calculating the linear response of finite and extended systems is TD-current-DFT (TDCDFT). In TDCDFT, the basic quantity is the total current-density of the system, rather than the density as in TDDFT, that has a one-toone mapping with an external vector potential. There are three main reasons for using TDCDFT: