The ring-polymer molecular dynamics method was used to calculate approximate Kubo-transformed velocity autocorrelation functions and self-diffusion coefficients for low-pressure liquid para-hydrogen at 25 and 14K. The diffusion coefficients were shown to be consistent with experimental shear viscosities and an established relationship between diffusivity, shear viscosity and length of the cubic simulation cell. The diffusion coefficients which were obtained in simulations using finite system sizes were therefore too small. Extrapolation to infinite system size corrected this deficiency and led to excellent agreement with experimental results. This demonstrated the influence of system-size effects upon quantum mechanical diffusion coefficients, and provided evidence that ring-polymer molecular dynamics was an accurate method for including quantum effects in condensed-phase molecular dynamics.

Quantum Diffusion in Liquid Para-Hydrogen from Ring-Polymer Molecular Dynamics. Miller, T.F., Manolopoulos, D.E.: Journal of Chemical Physics, 2005, 122[18], 184503