A classical mechanics technique for quantum linear response

It is shown that the lowest random phase approximation (RPA) excitation energies of a quantum many-fermion system can be obtained by minimizing an effective classical energy functional. The idea is based on an analogy between the RPA and classical Hamiltonian equations of motion. Generalized Lanczos recursion allows the minimum to be found very efficiently. The technique is used to find the electronic excitation spectrum of the C-60 molecule.