An ab initio path integral Monte Carlo simulation method for molecules   and clusters: application to Li_4 and Li_5^+
  A novel method for simulating the statistical mechanics of molecular systems in which both nuclear and electronic degrees of freedom are treated quantum mechanically is presented. The scheme combines a path integral description of the nuclear variables with a first-principles adiabatic description of the electronic structure. The electronic problem is solved for the ground state within a density functional approach, with the electronic orbitals expanded in a localized (Gaussian) basis set. The discretized path integral is computed by a Metropolis Monte Carlo sampling technique on the normal modes of the isomorphic ring-polymer. An effective short-time action correct to order $\tau^4$ is used. The validity and performance of the method are tested in two small Lithium clusters, namely Li$_4$ and Li$_5^+$. Structural and electronic properties computed within this fully quantum-mechanical scheme are presented and compared to those obtained within the classical nuclei approximation. Quantum delocalization effects are significant but tunneling turns out to be irrelevant at low temperatures. 
Ruben O. Weht, Jorge Kohanoff, Dario A. Estrin, and Charusita   Chakravarty (ICTP Trieste, Italy)