A core strength of the Theoretical Chemical and Quantum Physics Group

## Outline

In Molecular Mechanics a group of molecules can be considered as *classical* collections of balls and springs rather than *quantum* collections of electrons and nuclei. Models vary in complexity; the most simplistic case approximates atoms as hard spheres with a radius equal to that of their covalent radii, and then uses Valence Shell Electron Pair Repulsion Theory to predict the shape of molecules based upon the extent of the associated electron-pair electrostatic repulsion. This case is obviously trivial to calculate, and as this method is a classical approximation, terms tend to simply append to the original expression - rather than couple in a Gordian-esque knot like many quantum theories. Aided with modern computing power, even the most variegated system can be constructed in a fraction of the time it would take to generate the same system using Density Funcitonal Theory (DFT).

It has been shown that under homogeneous compression, solids can be described by an expression of pair-potentials and contributions from non-pair interactions can be expanded in terms of the pair interactions. This effectively includes any non-pair interactions into the pair terms and therefore the E(V) and P(V) states of the solid can be described on a unified basis. These interactions are mathematically constructed from well known classical mechanics formulae, and in Molecular Mechanics are known as *force fields*.

The CQP Group primarily uses The General Utility Lattice Program (GULP) when implementing Molecular Mechanics. GULP has the capability of simulating molecules and clusters (0D), polymers (1D), surfaces and slabs (2D), and bulk solids (3D). It uses empirical forcefield methods to simulate a variety of materials, such as the shell model for ionic systems, molecular mechanics for organic systems, embedded atom method (EAM) for metals, and reactive REBO potential for hydrocarbons.

GULP also allows the calculation of properties such as energies, bulk moduli, elastic constants, vibrational density of states, and heat capacity. In addition, it also allows one to perform NVE, NVT, and NPT molecular dynamics (MD) simulations. The CQP group typically uses GULP to perform tasks such as geometry optimisation of large systems, annealing of materials, and preparation of systems for further refinement using *ab-initio* methods.

For more information about this technique, please contact Jared Cole or Salvy Russo.