where ρ 0 (r) is the ground-state density and the functional derivative is evaluated to first order in u. This approximation was dubbed the "elastic approximation" in refs. 571,572 and reduces the problem of finding excitation energies to a standard eigenvalue problem with a dynamical matrix that is constructed from ground-state properties such as the pair correlation function and the one-particle density matrix. Only a few applications of this theory have been reported to date -see ref. 574 for a very recent one.