A force-calculation is to be run. The Hessian, that is the matrix in millidynes per Ångstrom) of second derivatives of the energy with respect to displacements of all pairs of atoms in x, y, and z directions, is calculated. On diagonalization this gives the force constants for the molecule. The force matrix, weighted for isotopic masses, is then used for calculating the vibrational frequencies. The system can be characterized as a ground state or a transition state by the presence of five (for a linear system) or six eigenvalues which are very small (less than about 30 reciprocal centimeters). A transition state is further characterized by one, and exactly one, negative force constant.
Before a FORCE calculation is run, the gradients are calculated to see if the geometry is at a stationary point. If it is not, then the calculation will be stopped, to allow the user to take corrective action.
Sometimes, the gradient norm at the start of a FORCE calculation will be larger than at the end of the geometry optimization which was used to generate the geometry for the force calculation. This is due to the FORCE calculation using a different method, double-sided derivatives, to calculate the gradients. In order to have the same GNORM at the end of a geometry optimization as at the start of a FORCE calculation, use PRECISE in the geometry optimization. Gradients calculated with PRECISE and with FORCE both use double-sided derivatives.
A FORCE calculation is a prerequisite for a THERMO calculation.
At the end of a FORCE calculation, the force constants for the coordinates supplied will be printed. If other force constants are needed, then use ISOTOPE to save the Hessian. The connectivity can then be changed, and the job restarted using RESTART. Of course care must be taken to ensure that the atoms are in exactly the same positions in both calculations.
Before a FORCE calculation is started, a check is made to ensure that a stationary point is being used. This check involves calculating the gradient norm (GNORM) and if it is significant, the calculation will be stopped. See also LET and TRANS. In a FORCE calculation, PRECISE will eliminate quartic contamination part of the anharmonicity). This is normally not important, therefore PRECISE should not routinely be used. In a FORCE calculation, the SCF criterion is automatically made more stringent; this is the main cause of the SCF failing in a FORCE calculation.