Diffusion Monte Carlo and PIMD calculations of radial distribution functions using an updated CCSD(T) potential for CH₅⁺

We report a new permutationally invariant polynomial (PIP) fit to CCSD(T)/aug-cc-pVTZ energies for CH5+ that provides fast analytical forces, unlike the original PIP fit of 2006. The new fit, which is slightly more precise than the previous one, is used in classical molecular dynamics, diffusion Monte Carlo and path integral molecular dynamics calculations of CH and HH radial distribution functions (RDFs) at temperatures of 10 and 50K. Corresponding RDFs are also reported for CD3H2+. The well-known quantum localisation of the H atoms to the diatomic site from previous and present DMC calculations is maintained at 10K but shows additional delocalisation to the ‘CH3+’ site at 50K.