Semiempirical Fragment Self-Consistent Field Monte Carlo simulations for liquid chlorosilanes

Abstract

We applied the recently developed Neglect of Diatomic Differential Overlap Fragment Self-Consistent Field Monte Carlo method to the simulation of the liquid state of chlorinated monosilanes. This semiempirical technique divides the periodic simulation cell into subsystem, where the random move of an atom takes place, and environment exerting only secondary effects on the former. Expanding the electronic wave function on the basis of atomic hybrid orbitals, that form strictly localised molecular orbitals corresponding to chemical bonds in classical molecules, the wave function of the environment is determined from coupled 2 × 2 secular equations. For the subsystem the conventional Self-Consistent Field equations, with a perturbation term in the Fockian and a much lower dimensionality than for the whole system, have to be solved. Thus the computational efforts drastically decrease as the dependence on the number of atoms in the environment reduces from quartic or cubic, as in conventional ab initio or semiempirical methods, to quadratic. Our simulation for chlorosilanes predicts that in the liquid state the preferred orientation of two neighbouring molecules is the one with the maximum number of SiH…ClSi hydrogen bonds. We found that gas phase SiCl bond lengths increase by 6 to 16 pm in the liquid state as a result of that association. HSiCl and ClSiCl bond angles change much less, by 2–4 degrees, as compared to the gas phase geometry. Since, to our knowledge, there are no experimental data published for these systems, our results may serve as preliminary information on liquid chlorosilanes.