Employing first principles parallel tempering molecular dynamics in the microcanonical ensemble, we report the presence of a clear solid-liquid-like melting transition in Al20(+) clusters, not found in experiments. The phase transition temperature obtained from the multiple histogram method is 993 K, 60 K above the melting point of aluminium. Root mean squared bond length fluctuation, the velocity auto-correlation function and the corresponding power spectrum further confirm the phase transition from a solid-like to liquid-like phase. Atoms-In-Molecules analysis shows a strong charge segregation between the internal and surface atoms, with negatively charged internal atoms and positive charge at the surface. Analysis of the calculated diffusion coefficients indicates different mobilities of the internal and surface atoms in the solid-like phase, and the differences between the environment of the internal atoms in these clusters with that of the bulk atoms suggest a physical picture for the origin of greater-than-bulk melting temperatures.

First principles molecular dynamics simulations of Ga19Al(+) have been performed in the microcanonical ensemble using parallel tempering. We perform a thorough investigation of the changes induced by the presence of an Al atom in the Ga dominated cluster. Dynamic analysis indicates that the Al atom prefers to occupy the internal sites of the cluster structure, at all temperatures, and above 450 K, the Al atom is less mobile than the central Ga atom throughout the simulation. Using the multiple histogram method, canonical specific heat curves are obtained that compare well with previous experimental measurements of the specific heat and equivalent simulations for the Ga20 (+) cluster. The first-principles melting temperature agrees well with the experimental value for Ga19Al(+). Analysis of the root mean squared fluctuation in bond length, velocity auto-correlation function, and the corresponding power spectrum, confirms the solid-liquid-like phase transition in Ga19Al(+), as for Ga20 (+).

Phase diagrams are the most reliable way of predicting phase changes in a system. In this work, we create the phase diagram of the Ga (20-x) Al x + bimetallic cluster system, for which the monometallic clusters display greater-thanbulk melting behaviour. Employing first-principles Born-Oppenheimer molecular dynamics in the microcanonical ensemble, we describe the solid-liquid-like phase transition in two distinct compositions Ga 11 Al 9 + and Ga 3 Al 17 + . The clusters show peaks in specific heat at 824 K and 922 K respectively which is above the corresponding bulk alloy melting temperatures. Mean squared displacements, diffusion coefficients and atoms-in-molecules analysis emphasise the differences between the environments of the internal and surface atoms. An Arrhenius-like description of the 'ease' with which internal and surface sites (or atoms) can exchange positions with temperature is used to quantify the greater-than-bulk melting character which corroborates the view of a Ga (20-x) Al x + cluster being a remnant of the corresponding bulk alloy material with additional characteristics specific to its size and composition.