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4 Free Energy enthalpies of crystals from measurements of their SVP. The latter measurement is however a difficult one, because these pressures are of the order of 10–5 to 10–10 bar. Uncertainties on measured sublimation enthalpies are never smaller than 5–10 kJ mol–1, and this should be kept in mind when using these numbers to calibrate intermolecular potentials. Free energies can be simulated by the so-called free energy perturbation or thermodynamic integration methods. While, in principle, more revealing than enthalpy-based simulations, free energy simulations have a number of shortcomings.

Quite similar results are obtained by high-level ab initio calculations including electron correlation. g. the aromatics) and ð-electron clouds of benzenoid rings. Of even lesser importance are most C–H···O or C–H···N type interactions, worth just a few kJ mol–1, while other interactions (like Cl···Cl contacts, often invoked as important in crystal structures) are uninfluential. 3 shows that only the hydrogen bond has a relatively narrow width, corresponding to a really competitive binding power.

1) in order to reproduce the main trends obtained in accurate calculations. Here we describe those presently thought to be most reliable. 2) where K is a proportionality constant that depends on the specific interacting molecules. 3) Its exponential behavior makes this term the dominant one when short atom–atom distances between the interacting molecules are produced. Consequently, this term prevents molecules from getting closer than some limiting distance; this is the physical principle behind the hard-sphere model and Kitaigorodsky’s close-packing principles [2].