The VCHam program uses internally dimensionless mass-frequency-scaled (DMFS) normal modes.

After reading the parameter file, the VCHam program builds up the matrices for the transformation between the Cartesian coordinates and the DMFS normal modes.

The reading of the calulated ab initio energy values onto the PES and their position (in Cartesian coordinates) follows.

To derive the VC model prameters, the VCHam program uses a least-square fit routine, to minimize the squared difference of the VC model PES and the ab initio based PES.