Inclusion of the ionic zero-point motion in the density functional theory-beyond the Born-Oppenheimer approximation

We introduce a novel method to carry out a zero temperature density functional theory (DFT) calculation for systems in which the ionic motion is treated fully quantum mechanically and the electronic degrees of freedom are treated within imaginary-time dependent DFT. The approach is based on the finite-temperature many-body path-integral formulation of quantum mechanics by taking the zero-temperature limit and treating the imaginary-time propagation of the electronic variables ``exactly'' within the DFT. Our approach goes beyond the familiar Born-Oppenheimer approximation and is limited from being exact only by the approximations involved in the DFT approach and it includes the effects of the electronic charge correlations in imaginary time. The method is tested in simple molecules which contain light atoms, such as hydrogen, where the fluctuations of the distance from its equilibrium position, due to the zero-point-motion, is comparable to the interatomic distances.