Extraction of analytical potential function parameters from ab initio potential energy surfaces and analytical forces. The program PAROPT has been written to extract forcefield parameters from ab initio calculations of potential energy surfaces (PES) and ab initio analytical forces. The ability to use either energies or forces, or a combination of the two to determine forcefield parameters is a novel feature of the program. Simulated annealing is used within the program to minimise the difference between a set of forces and energies calculated using ab initio techniques and the same quantities calculated using an empirical forcefield. Details of the program and methods used to extract forcefield parametrizations are presented.