Forces in the benzene crystal

The total lattice energy of benzene is evaluated by summation of pair interactions in the crystal. The relative contributions of the σ and π electrons to the attractive part of the energy are separated out explicitly: the π electrons contribute 40 per cent of this energy (although they represent only 20 per cent of the valence electrons). However the three component σ-σ, σ-π, and π-π dispersion energies are of the same order of magnitude. The effect of σ-π and π-π electron correlation is also investigated. The total lattice energy agrees well with the available experimental heat of sublimation data.