Description

In AMPAC 8, the translational and rotational “vibrations” are computed analytically and the force constant matrix is projected from the subspace of the genuine vibrations. This corrects the harmonic force field by removing the residual first order components present because the system is not exactly a stationary point. The geometry must be very near a stationary point so that third and higher order components do not corrupt the quadratic force field. Before a FORCE calculation can be performed the gradient norm must very low. If the gradient norm is too large, the calculation will be halted. This condition may be overridden by use of the keyword LET. See Chapter 4, Computational Procedures for a discussion of stationary points and the implications of incompletely optimized geometries.