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

摘要

An ab initio Hartree-Fock molecular dynamics procedure is applied to study structural and dynamical properties of Li-9(+), Li-10, and Li-11(+) clusters with eight and ten valence electrons, corresponding to ''closed'' and ''open'' shell systems. Gradients of the ground state energy are used to compute the forces acting on atoms at each geometric configuration along trajectories generated by solving classical equations of motion. Dynamics of different isomers for each cluster size have been investigated as a function of excess energy. It is shown that different isomers, even those similar in energy, can exhibit different structural 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 peripheral atoms at relatively low excess energy. In contrast, compact tetrahedral type structures show much more rigid behavior at low excess energy. However, the former ones need larger excess of internal energy to undergo isomerizations to geometrically different structures than the latter ones, at least in the case of Li-9(+) and Li-11(+) clusters. At the time scale of our simulations we found that for the intermediate excess energies it is ''easier'' to carry the duster in the basin of the lowest energy isomer than in the reverse direction. Moreover, for different cluster sizes isomerization processes occur at different excess energies (temperatures), which is a consequence of the differences in the structural properties rather than in the number of the valence electrons. It has been found that the liquidlike behavior in small Li clusters becomes apparent at relatively high temperature in spite of large mobility of their atoms. (C) 1997 American Institute of Physics. [References: 52]