Molecular dynamics studies of gold: bulk, defects, surfaces and clusters

In recent years, computer simulation methods have provided much insight into several structural, dynamical and thermal properties of solids and liquids. Computational methods are particularly well suited to the study of low symmetry sys.terns (e.g., defects, surfaces, clusters), where the complexity of analytical treatments may become overwhelming, and of systems at finite temperature. The key ingredients in computer simulations are interatomic forces. The problem we wish to solve can be simply stated as follows: given a set of N atoms having some positions r 1 •.. rN and linear momenta p1 .•• PN, what forces will they experience ? Calculating these forces ab initio is a very difficult task. Even if we are not interested in the electronic· properties of the system, but only in ionic properties (e.g., vibrations, equilibrium structures, etc.), we must generally take into full account the electronic aspect of the problem. In the Born-Oppenheimer adiabatic approximation [1 J, the forces can be obtained by considering the nuclei as fixed and searching for the minimum energy state of the electronic system. This .may be done using the Hartree-Fock approximation, or in a density functional theory framework. The force acting on a nucleus is then determined as the gradient of the total energy respect to a displacement of that nucleus. After all nuclei have been moved accordingly to the forces computed in _this way, the whole process may be iterated for the new configuration, ·thus performing a dynamical calculation. This approach, however, is computationally extremely expensive, and not feasible when the number of particles is of the order of ten or more....