  We propose an efficient and general strategy of metadynamics (MetaD) for
investigating interstitial diffusion in a crystal by exploiting
crystallographic symmetry. Assuming complete ignorance of the diffusion
phenomenon of interest, the three-dimensional coordinates of the interstitial
atom with the periodic boundaries are chosen as the collective variables (CVs).
Multiple potential hills are simultaneously deposited at all
crystallographically-equivalent positions on the free energy surface (FES)
defined in the CV space. As a result, the proposed multi-hill strategy highly
accelerates atomic jumps in comparison with the single-hill strategy in the
conventional MetaD. The key features are that the FES estimated from the final
bias potential is exactly satisfied with the symmetry of the host crystal and
that all elementary processes of interstitial diffusion are obtained by the
single MetaD simulation without any prior knowledge on the diffusion mechanism.
The high efficiency and efficacy of the multi-hill strategy are demonstrated,
taking the proton diffusion in barium zirconate with the cubic perovskite
structure as a model case.
