The OPT program reads the geometry definitions and geometry parameters, energies, and gradients of the present and previous points from the geometry record 700.1. The program then predicts a new optimum geometry (i.e. takes one optimization step), by default optimizing all variable parameters on which the geometry depends, and writes this back to the geometry record on file 1. The optimization is performed in a space of scaled parameters, the scaling being such that the initial hessian matrix has unit diagonal elements. The variable OPTCONV is set to the length of the step taken in scaled parameter space, and can be tested after the OPT step using the IF command, to decide whether to return for another geometry. For subcommands of OPT see OPTG.