Systematic determination of MCSCF equilibrium and transition structures and reaction paths

The restricted step optimization algorithm is applied to potential energy surfaces calculated from multiconfiguration self‐consistent‐field wave functions. Equilibrium and transition‐state geometries are determined by iteratively solving a set of level‐shifted Newton–Raphson equations. At each geometry the molecular gradient and Hessian are calculated analytically, and a first‐order prediction of the wave function at the next geometry is obtained by combining the geometrical derivatives of the wave function with the geometrical step vector. The usefulness of this prediction is discussed and illustrated by test calculations. The numerical accuracy which is required in the wave function and its geometrical derivatives in order to maintain quadratic convergence in the optimization of the molecular geometry is analyzed. It is demonstrated that the Newton–Raphson step vector and the wave function prediction may be determined without calculating the molecular Hessian explicitly. Sample calculations are carried ...