The force field calculation can feed directly
into calculating vibronic energy levels and transitions, or either
mode can be used in a stand alone manner. Either (or both)
calculation can be worked backwards to fit observed values to
molecular parameters. Both modes can use ab initio
calculations as a starting point, and even low quality ab
calculations can give a useful starting point for
analysis. A typical use case might involve an ab initio
calculation of the force field or other parameters, which is then
adjusted or refined to experimental observation or other criteria.
To start, work through the worked examples below.