  Mass tensor molecular dynamics was first introduced by Bennett [J. Comput.
Phys. 19, 267 (1975)] for efficient sampling of phase space through the use of
generalized atomic masses. Here, we show how to apply this method to ab initio
molecular dynamics simulations with minimal computational overhead. Test
calculations on liquid water show a threefold reduction in computational effort
without making the fixed geometry approximation. We also present a simple
recipe for estimating the optimal atomic masses using only the first
derivatives of the potential energy.
