Centroid path integral molecular-dynamics studies of a para-hydrogen slab containing a lithium impurity

Six-layer para-hydrogen (p-H-2) semi-infinite slabs, containing a Li impurity atom in different locations, have been investigated at 2.5-14.7 K by means of centroid path integral molecular-dynamics simulations. On the time scale of the simulations (15 ps), lithium atoms initially doped inside the slab appear to remain trapped in the lattice and show no tendency to diffuse toward the surface. The slab surface layers are observed to melt more readily than the inner ones, even when the dopant Li atom is initially placed in the interior. In some cases, before melting occurs, p-H-2 molecules are ejected onto the slab surface and diffuse two dimensionally. Lithium is promoted to the surface only when it is initially buried in the surface layer or when the whole slab melts and a diffusion pathway is opened toward the surface. (C) 1998 American Institute of Physics.