On the classical limit for electronic structure and dynamics in the orbital approximation

The classical limit is shown to provide a description exactly equivalent to the quantum mechanical one in the approximation where each electron is ass igned to an orbital. Strictly speaking it is therefore not a limit but an a lternative way of solving the problem. There are some merits of this reform ulation, most notably in that it brings the phase of the orbitals to the fo refront, on equal footing as the occupancies. This allows one to discuss, e .g., electron localization, in a clearer manner. But computationally the cl assical description is not superior. There will be a definite advantage for more realistic electronic Hamiltonians, i.e., for implementing configurati on interaction, and/or when the nuclear motion is coupled to the electronic dynamics. In this paper we limit attention to a derivation and discussion of the simple orbital approximation. (C) 2000 American Institute of Physics . [S0021-9606(00)30435-4].