On the Dirac-Frenkel/McLachlan variational principle

The Dirac-Frenkel/McLachlan variational principle (DFMVP) is re-derived usi ng a general and strict formalism which is valid for ail practical applicat ions. General equations of motion for the parameters are derived from the D FMVP. The time-dependent parameter set solving the equations of motion form s an approximate variational solution of the original problem. An error est imate for the approximation is presented which shows that the variational s olution converges to the exact one if the parametrisation is improved. (C) 2000 Elsevier Science B.V. All rights reserved.