First principles path integral simulations of nuclear quantum effects

by Kjartan Thor Wikfeldt (Nordita)

Monday 29 Oct 2012, 10:15 → 11:15 Europe/Stockholm

FA32

Hydrogen atoms are known to exhibit significant nuclear quantum effects (NQEs) which affect the strength of hydrogen (H-) bonds in systems ranging from biological molecules and ferroelectric crystals to high-pressure ice. The rate of diffusion of hydrogen in both the bulk and on the surface of materials is also greatly enhanced by NQEs due to the possibility of quantum tunneling. A powerful method to study NQEs is based on Feynman’s path integrals, where atomic nuclei are treated as ring-polymers instead of the commonly used approximation of point-like nuclei. By combining path-integral molecular dynamics (PIMD) with density functional theory (DFT), complex systems can be described at a completely quantum mechanical level. I will present results from ab initio PIMD simulations of two interesting systems where nuclear quantum phenomena play an important role. The first project concerns diffusion of H on the Ru(0001) surface, where experimental colleagues have measured the jump rate of H down to low temperatures and found a crossover to tunneling-dominated diffusion. The second projects concerns squaric acid, an organic crystal belonging to the KDP-type family of H-bonded ferroelectrics. Squaric acid displays a giant isotope effect on the antiferroelectric-paraelectric phase transition temperature, indicating substantial quantum effects, and our simulations have shed light on the underlying mechanisms behind this behavior.