Force constants are most commonly derived from infrared spectra, which is naturally well suited to describing force constants. 434.0 1.010 VDW parameters can likewise be derived from analysis of x-ray and neutron diffraction data. The strictly geometric properties are well understood and well described by force fields. The geometric terms do not vary a great deal from force field to force field since they are all derived from the same common pool of experimental data. Unfortunately, the geometric terms are only a minor consideration compared to the electrostatic interactions which are not so well described.
Molecules are collections of nuclie which share common electrons.
The nuclie have charges equal to the sum of their protons. In force fields, molecules are represented as a collection of fixed charged point masses. Assigning charges to these point masses is not a completely resolved issue. Charge assingment methodology differs from force field to force field. The Gromos87 charges are the simplest possible and are the most rational from the mentality of general chemistry.