AB-INITIO MOLECULAR-DYNAMICS STUDY OF SOLID-TO LIQUID-LIKE TRANSITIONS IN LI-9(-10, AND LI-11(+) CLUSTERS(), LI)

An ab initio Hartree-Fock molecular dynamics procedure is applied to s tudy structural and dynamical properties of Li-9(+), Li-10, and Li-11( +) clusters with eight and ten valence electrons, corresponding to ''c losed'' and ''open'' shell systems. Gradients of the ground state ener gy are used to compute the forces acting on atoms at each geometric co nfiguration along trajectories generated by solving classical equation s of motion. Dynamics of different isomers for each cluster size have been investigated as a function of excess energy. It is shown that dif ferent isomers, even those similar in energy, can exhibit different st ructural and dynamical behavior. The analysis of the simulations leads to the conclusion that structures with a central atom, in particular the centered antiprism of Li-9(+), exhibit concerted mobility of the p eripheral atoms at relatively low excess energy. In contrast, compact tetrahedral type structures show much more rigid behavior at low exces s energy. However, the former ones need larger excess of internal ener gy to undergo isomerizations to geometrically different structures tha n the latter ones, at least in the case of Li-9(+) and Li-11(+) cluste rs. At the time scale of our simulations we found that for the interme diate excess energies it is ''easier'' to carry the duster in the basi n of the lowest energy isomer than in the reverse direction. Moreover, for different cluster sizes isomerization processes occur at differen t excess energies (temperatures), which is a consequence of the differ ences in the structural properties rather than in the number of the va lence electrons. It has been found that the liquidlike behavior in sma ll Li clusters becomes apparent at relatively high temperature in spit e of large mobility of their atoms. (C) 1997 American Institute of Phy sics.