We present an efficient method to mix well converged ab initio forces with simpler and faster ones in molecular dynamics. While the cheap forces are evaluated every time step, the converged ones correct the trajectory only every n time steps. For convenience, both types of forces are calculated with the same basic scheme, using density functional theory, norm conserving pseudopotentials, and a basis set of numerical atomic orbitals. The cheap forces are evaluated with a short-range minimal

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... set and the non-selfconsistent Harris functional. Since these evaluations are hundreds of times faster than those of the converged forces, they add a neglegible cost, and the boost in computational efficiency is approximately a factor n. Our results indicate that one can use values of n of up to 10, without affecting significantly the calculated structural and dynamical magnitudes.