  We present a simple and accurate computational method, which facilitates
ab-initio path-integral molecular dynamics simulations, where the quantum
mechanical nature of the nuclei is explicitly taken into account, at
essentially no additional computational cost in comparison to the corresponding
calculation using classical nuclei. The predictive power of the proposed
quantum ring-polymer contraction method is demonstrated by computing various
static and dynamic properties of liquid water at ambient conditions. This
development permits to routinely include nuclear quantum effects in ab-initio
molecular dynamics simulations.
