Orbital-free molecular dynamics simulations of melting in Na_8 and Na_20 : Melting in steps

Abstract

The melting-like transitions of Na_8 and Na_20 are investigated by ab initio constant energy molecular dynamics simulations using a variant of the Car–Parrinello method which employs an explicit electronic kinetic energy functional of the density, thus avoiding the use of one-particle orbitals. Several melting indicators are evaluated in order to determine the nature of the various transitions, and are compared with other simulations. Both Na8 and Na_20 melt over a wide temperature range. For Na_8, a transition is observed to begin at ∼110 K, between a rigid phase and a phase involving isomerizations among the different permutational isomers of the ground state structure. The “liquid” phase is completely established at ∼220 K. For Na_20, two transitions are observed: the first, at ∼110 K, is associated with isomerization transitions among those permutational isomers of the ground state structure which are obtained by interchanging the positions of the surface-like atoms; the second, at ∼160 K, involves a structural transition from the ground state isomer to a new set of isomers with the surface molten. The cluster is completely liquid at ∼220 K.