ON THE USE OF A HESSIAN MODEL FUNCTION IN MOLECULAR-GEOMETRY OPTIMIZATIONS

When a molecular equilibrium geometry is determined by minimizing the energy by a quasi-Newton-Raphson method, the number of iterations requ ired depends critically on the choice of an approximate molecular Hess ian matrix. We find that a simple 15-parameter function of the nuclear positions gives a good choice for any molecule with atoms from the fi rst three rows of the periodic table. This Hessian is used for ah init io geometry optimizations with the quasi-Newton-Raphson method, with o r without update. The equilibrium geometries of 30 molecules, with a v ariety of sizes and symmetries, is obtained with the new scheme, which is shown to converge significantly faster than other methods.