Quantitative Ranking of Crystal Packing Modes by Systematic Calculations on Potential Energies and Vibrational Amplitudes of Molecular Dimers.

Quantum chemical calculations including electron correlation and calculations with the density sums (Pixel) method have been performed on a variety of molecular dimers representing some frequently observed recognition modes in molecular condensed phases. Notwithstanding some individual fluctuations when different computational methods are used, there is a general agreement for the relative orders of magnitude. The results have been collected in a table that ranks the interaction energies and amplitudes of the energy wells for the recognition between molecular fragments, providing a quantitative guideline to assess the relative importance of hydrogen bonding, aromatic ring stacking, antiparallel arrangements of polar moieties, weak Coulombic C-H···X interactions, and dispersive interactions between nonpolar groups. Since the Pixel method naturally allows for a separation between Coulombic, dispersion, polarization, and repulsion energy contributions, their relative importance can be analyzed with respect to the chemical constitution of the interacting partners. The relevance of these results to the current ideas and methods of "crystal engineering" is discussed.