Notes on what else besides geometry can be in a model

Any useful information, which we can predict about the molecule, can be part of the model

All computer modelling methods calculate the energy of a single molecule
This translates most nearly, per mole, into the thermodynamic internal energy U
None of the common methods give absolute energy accurate enough to be useful
Differences in energy between similar molecules, calculated by the same method, can be useful. This course will examine which methods give energy differences accurate enough for which purposes.
Relative energies allow us to calculate the energies of some reactions
If we ignore entropy change and make the approximation DG = DU then we can obtain equilibrium constants
These are particularly useful for equilibria between conformers, and rotamers in particular
Predicted equilibrium constants allow, e.g.:

prediction of relative conformer concentrations, hence which reaction dependent on them is most likely
assignment of which observed integral in the low temperature NMR spectrum belongs to which conformer

All modelling methods give force constants for bond stretching, angle deformation, etc.
These allow IR and Raman frequencies to be predicted
For some methods and some molecules, these may be good enough to allow relative assignment of IR spectra to two observed molecules

Electronic structure, i.e. MO, methods calculate the distribution of electrons
It is often hard to get enough accuracy to do a good job of predicting properties which depend directly on electron distribution, e.g. atomic charges, or dipole moments which can be compared with experiment
Many biological interactions are essentially electrostatic, and depend on charge distributions, so there is great interest in calculating electrostatic potential