Dear Stefano and Axel, thanks for your replies. Actually question can be rephrased as follows. Thanks to the Hellmann-Feynman theorem in order to compute the ionic forces I need only the partial derivatives of the total energy with respect to the ionic coordinates (see for instance 7.5 and 7.8 in Payne et al., RMP 64, p. 1045). If no real displacements of the atoms are needed to compute these derivatives, then I guess that they can be simply calculated as expectation values of the Hamiltonian on the current wavefunction because they can written down once a (parametrized) expression of the ionic potential is chosen. Am I right?
Thanks for still devoting time to this... -- Cristian Degli Esposti Boschi CNR-IMM, Sezione di Bologna, via Gobetti, 101, 40129, Bologna, Italia tel. ++39 051 6399152, fax ++39 051 6399216 degliesposti -AT- bo.imm.cnr.it
