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 s ystems in which both nuclear and electronic degrees of freedom are tre ated quantum mechanically is presented. The scheme combines a path int egral description of the nuclear variables with a first-principles adi abatic 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 C arlo sampling technique on the normal modes of the isomorphic ring pol ymer. An effective short-lime action correct to order tau(4) is used. The validity and performance of the method are tested by studying two small lithium clusters, namely Li-4 and Li-5. Structural and electroni c properties computed within this fully quantum-mechanical scheme are presented and compared to those obtained within the classical nuclei a pproximation. Quantum delocalization effects turn out to be significan t as shown by the fact that quantum simulation results at 50 K approxi mately correspond to those of classical simulations carried out at 150 K. The scaling factor depends, however, on the specific physical prop erty, thus evidencing the different character of quantum and thermal c orrelations. Tunneling turns out to be irrelevant in the temperature r ange investigated (50-200 K). (C) 1998 American Institute of Physics. [S0021-9606(98)51421-3]