The effect of using a realistic model for the electrostatic forces on the calculated structures of molecular crystals is explored by using atomic multipoles derived from an SCF 6-31G** wave function. This was tested on a wide ranging database of 40 rigid organic molecules containing C, H, N, and O atoms, including nucleic acid bases, nonlinear optic materials, azabenzenes, nitrobenzenes, and simpler molecules. The distributed multipole electrostatic model, plus an empirical repulsion-dispersion potential, was able to successfully reproduce the lattice vectors and available heats of sublimation of the experimental room temperature structure in almost all cases. Scaling the electrostatic energy to allow for the effect of electron correlation on the molecular charge density generally improved the lattice energies and the calculated structures to a lesser extent. However, omitting the anisotropic multipole moments usually gave very poor, sometimes qualitatively wrong structures, emphasizing the sensitivity of these crystal structures to the electrostatic forces. We also investigated the sensitivity of the structures to the empirical repulsion-dispersion potential parameters by attempting to optimize these. Since the experimental structures are mainly reproduced to within the errors that could be attributed to the use of static minimization and rigid molecules, it appears that going beyond the atomic charge model to a realistic electrostatic model is a key development in the modeling of the crystal structures of polar and hydrogen-bonded molecules.

We examine the fundamental processes involved in the hydrothermal synthesis of silicates, using a range of computational techniques. We report ab initio calculations of the structures and energies of silica fragments. We estimate hydration energies by using both molecular mechanics methodologies and dielectric screening techniques. Detailed molecular mechanics calculations are reported for the interactions between solvated fragments of zeolitic structures and template molecules, emphasizing the role of the template in modifying fragments so that their structures are closer to those observed in zeolite crystals.

Density functional theory (DFT) and force-field-based calculations have been carried out on the breathing metal-organic framework MIL-53(Cr) in both its large-and narrow-pore forms. In its sorbate-free form, the large-pore structure appears to be the global minimum. We develop a hybrid force field combining ionicmodel potentials, used for modeling inorganic solids, with molecular mechanics terms for the organic part. This gives an energy difference of close to 30 kJ mol -1 between the large-and narrow-pore forms. Calculations in which water molecules are introduced into the structures illustrate how the energetics of physisorption are able to drive the pore breathing process, with the water molecules, being more strongly stabilized in the narrow-pore form, favoring pore closure at loadings of more than one molecule per unit cell.