General Utility Lattice Program Version 4.0 Julian D. Gale Nanochemistry Research Institute, Department of Chemistry, Curtin University, P.O. Box U1987, Perth, WA 6845, Australia email: gulp@ivec.org 1 Chapter 1 Introduction & background The General Utility Lattice Program (GULP) is designed to perform a variety of tasks based on force field methods. The original code was written to facilitate the fitting of interatomic potentials to both energy surfaces and empirical data. However, it has expanded now to