DFPT is an analytical approach to compute the response of a molecule or material to an external electric (or magnetic) field. It can be viewed as the solid-state formulation of the coupled perturbed self-consistent field (CPSCF) method originally developed in the quantum chemistry community. The derivation of the DFPT equations relies on perturbation theory. The KS Hamiltonian is modified by an additional term ĥE = -r • E, where E is the applied homogeneous electric field. E E denotes the energy term due to ĥE .