The NH3-NH3 intermolecular interaction: the (NH3)2-5 small ammonia clusters and liquid ammonia

Abstract

Molecular Dynamics (MD) is a simulation technique that allows to analyse the interactions between atoms and molecules along the time, giving a visualization of the movement of the particles. For this simulation, an energy potential model describing the interaction needs to be implemented in MD programmes, giving us information directly comparable with experimental behaviour. MD simulations not only are useful for comparative purposes but also for predictive ones. Obviously, if we want to make predictions with a high guarantee of success, the model must be accurate. However, in order to reduce the simulation computational cost, the model has to be as simple as possible. Ammonia is one of the most important compounds in nature because it contributes significantly to the nutritional needs of living organisms. In recent years, there have been a large number of accurate theoretical as well as experimental investigations on small ammonia clusters interactions, but not so many about liquid ammonia. In order to represent big systems with lots of molecules (as required to simulate liquids), it is very important to have a function describing as good as possible the intermolecular interactions. In MD the previous interaction is often expressed as a sum of electrostatic and non electrostatic interaction contributions. The electrostatic one is usually defined by the Coulomb’s law equation, whereas the non electrostatic one is often given by the Lennard Jones (LJ) potential. The LJ function, although it is quite good and widely used in MD simulations, it presents some problems. It is too repulsive at short distances, and too attractive at long distances. For this reason, the present TFG focuses on the study of ammonia using a modified LJ potential energy function (ILJ) to describe the non electrostatic contribution. The ammonia study, according to the different properties of small clusters and liquid ammonia, has been divided in two parts. In the first one, the binding energy and equilibrium geometry of some small clusters, (NH3)2-5, have been calculated using the ILJ function and the results have been compared with other theoretical as well as experimental data. In the second part, once the reliability of the potential energy function has been proved, it has been applied to investigate some characteristics of liquid ammonia. In particular, the evolution of the density values and of the diffusion coefficients with the temperature has been analysed. Moreover some structural properties of liquid ammonia have been compared with X-ray and neutron diffraction experimental data.