Path integral Monte Carlo studies of the H5+/D5+ clusters using ab initio potential surfaces

We report here on classical and path integral Monte Carlo studies for the H-5(+) cluster and its deuterated counterpart, in order to investigate the floppy nature of its molecular structure due to anharmonic quantum effects. This method relies on the standard harmonic normal mode analysis and has been found to be effective for evaluating thermochemical/ground- state properties of highly anharmonic systems. A full-dimensional recent analytical CCSD(T) potential surface and a novel realistic density functional theory (DFT) 'on the fly'-based potential scheme were employed. Thermal equilibrium energies for H-5(+) and D-5(+) are determined from the path integral Monte Carlo (PIMC) calculations. The H-5(+) and D-5(+) probability density distributions are also obtained from both classical Monte Carlo and fully converged PIMC calculations, and they show strong spatial delocalization with highly anharmonic character. It was found that, on average, H-5(+) and D-5(+) can be described as a proton shared between the two outer almost freely rotating H-2/D-2 molecules. The implementation of such a combined PIMC/DFT approach to study nuclear quantum fluctuation on the electronic properties of H-5(+) is discussed, and its extension to larger protonated hydrogen clusters is also proposed.