Path-integral (PI) molecular simulations can be used to calculate exact quantum statistical mechanical properties for complex systems containing many interacting atoms and molecules. The limiting computational factor in a PI simulation is typically the evaluation of the potential energy surface (PES) and forces at each ring-polymer "bead"; for an n-bead ring-polymer, a PI simulation is typically n times greater than the corresponding classical simulation. To address the increased computational effort of PI simulations, several approaches have been developed recently, most notably based on the idea of ring-polymer contraction (RPC) which exploits either the separation of the PES into short-range and long-range contributions or the availability of a computationally-inexpensive PES which can be incorporated to effectively smooth the ringpolymer PES; neither approach is satisfactory in applications to systems described by ab initio PESs. In this Article, we describe a new method, ring-polymer interpolation (RPI), which can be used to accelerate PI simulations without any prior assumptions about the PES. In simulations of liquid water under ambient conditions, where quantum effects are known to play a subtel role in influencing experimental observables such as diffusion coefficients and radial distribution functions, we find that RPI can accurately reproduce the results of fully-converged PI simulations, albeit with far fewer PES evaluations; this approach therefore opens up the possibility of large-scale PI simulations on ab initio PESs at lower computational effort than current methods.

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