In the current funding period we have made advances on three fronts: (1) use of kinetic Monte Carlo (kMC) simulations to predict the morphology of organic crystals grown from solution for cases where the solution is pure and also when it contains growth inhibitor molecules, (2) completed incorporating three new all-atom force fields into ADDICT in order to test the sensitivity of our morphology predictions with respect to different estimates of intermolecular interaction energy, and (3) developed a new crystal growth model for asymmetric organic molecules with two molecules in the unit cell. This is a precursor to developing a more general model with many molecules in the unit cell. We report progress on all three topics.
We have continued to develop our kinetic Monte Carlo (kMC) modeling codes to predict the morphology of organic crystals grown from solution both with and without the inclusion of impurity molecules on the crystal surfaces. We have used these codes to make morphology predictions for naphthalene grown from ethanol solvent at increasing supersaturations in impurity-mediated solutions. The results were quite satisfactory and are reported in more detail later in the report.
In addition to the Generalized Amber Force Field (GAFF) which is already included in ADDICT, we have added three new all-atom force fields to ADDICT. They are the Coulomb-London-Pauli (CLP) force field, the Consistent Force Field (CFF), and the Universal Force Field (UFF). Each has its own specific advantages and limitations. CLP is the most general with over 90 atom types (similar to GAFF). CFF is specific to organic molecules containing intermolecular hydrogen bonds, especially carboxylic acids and amines. UFF has one atom type for every element in the periodic table - it is very general but with only one atom type for each element it is not very accurate. Later in the report we describe these force fields in more detail and show new results for predicting the morphology of five organic crystals using the CLP force field. We continue to develop a new crystal growth model for asymmetric organic molecules under the restriction of that the unit cell contains exactly two molecules. This allows us to make a big leap from essentially one (symmetric) molecule per unit cell to two asymmetric molecules. Once this theory is fully tested and validated on real molecular systems it will lead to further extensions to 4 molecules in the unit cell, and then many.