Finite temperature electronic simulations beyond the Born-Oppenheimer approximation
We introduce a general technique to compute finite temperature electronic properties by a novel covariant formulation of the electronic partition function. By using a rigorous variational upper bound to the free energy we are led to the evaluation of a partition function that can be computed stochastically by sampling electronic wave functions and atomic positions (assumed classical). In order to achieve this target we show that it is extremely important to consider the non trivial geometry of the space defined by the wave function ansatz. The method can be extended to any technique capable to provide an energy value over a given wave function ansatz depending on several variational parameters and atomic positions. In particular we can take into account electronic correlation, by using the standard variational quantum Monte Carlo method, that has been so far limited to zero temperature ground state properties. We show that our approximation reduces correctly to the standard Born-Oppenheimer (BO) one at zero temperature and to the correct high temperature limit. At large enough temperatures this method allows to improve the BO, providing lower values of the electronic free energy, because within this method it is possible to take into account the electron entropy. We test this new method on the simple hydrogen molecule, where at low temperature we recover the correct BO low temperature limit. Moreover, we show that the dissociation of the molecule is possible at a temperature much smaller than the BO prediction. Several extension of the proposed technique are also discussed, as for instance the calculation of critical (magnetic, superconducting) temperatures, or transition rates in chemical reactions.