Dialog Box: Convert Cartesian to Internal [Control window: Forces menu]
This option converts the forces in a Cartesian matrix, from a file specified in the Cartesian Forces from File dialog, to valence forces as specified in other dialogs in the Forces menu. When the conversion is completed, you have the option of replacing the current valence force constants with the derived values, and the results will be printed at the end of the output in the Output Window.
The order of atoms in the Cartesian force matrix must correspond to the generated atoms which are printed in the output. If you intend to use the Cartesian force output from a molecular-orbital program, it may be necessary to use the atoms generated by VIBRATZ as input to that program, as the sequence of generated atoms in VIBRATZ is dictated by symmetry.
This conversion will be valid only to the extent that you have specified a complete set of forces in the other dialogs of the Forces menu. The actual force constants entered in these dialogs will not be used in the conversion; any such values will be replaced at the end of the calculation, at your option. However, it is probably best to do a preliminary frequency calculation using the valence forces with nominal force-constant values, just to be sure that there are no non-zero frequencies (if there are, the forces are incomplete). Also, you can test that VIBRATZ that the conversion is correctly functioning by writing a Cartesian force file, and using that as input.
See the Import GAUSSIAN File dialog for a step-by-step example of the procedure for importing and converting a molecular-orbital file.
Using a Urey-Bradley force field for conversion is valid, but may lead to anomalous results. The strong correlation between angle and ligand-ligand forces may result in negative force constants for either.