Predicting the crystal structure of organic molecular materials

Abstract

This paper describes a novel method for predicting the crystal structure of organic molecular materials which employs a series of successive approximations to focus on structures of high probability, without resorting to a brute force search and energy minimization of all possible structures. The problem of multiple local minima is overcome by assuming that the crystal structure is closely packed, thereby eliminating 217 of the 230 possible space groups. Configurations within the 13 remaining space groups are searched by rotating the reference molecule about Cartesian axes in rotational increments of 15 ° . Initial energy minimization is performed using (6-12) Lennard-Jones pair potentials to produce a set of closely packed structures. The structures are then refined with the introduction of a Coulombic potential calculated using molecular multipole moments. This method has successfully located local minima which correspond to the observed crystal structures of several saturated and unsaturated hydrocarbons with no a priori information provided. For large polycyclic aromatic hydrocarbons, additional refinements of the energy calculations are required to distinguish the experimental structure from a small number of closely packed structures. Our methodology for a priori crystal structure prediction represents the most efficient algorithm presented to date, in a field where the first successes have only been described within the past year and have been few and far between. Since our algorithm is capable of locating a large number of reasonable structures with similar energy in a short period of time, and is more likely to locate a minimum corresponding to the experimental structure, our program provides a superior framework to determine the level of theory required to calculate the intermolecular potential. For all but highly asymmetric hydrocarbons, however, distinguishing the observed structure from a large number of highly probable structures requires more rigorously calculated intermolecular interactions than pair potentials, plus an ad hoc electrostatic potential, and is thus beyond the scope of this paper. All calculations 1" Current address: The

Electron Crystallography of Small Organic Molecules: Criteria for Data Collection and Strategies for Structure Solution

“…This approach soon found a molecular packing motif corresponding to a lattice energy of -130.40 kcal mol -I. [This is identical to the value found in an earlier ab initio prediction of this structure (Chaka et al, 1996) and is lower than the packing energy calculated from the X-ray crystal structure, i.e.…”

“…If the determination is constrained to an envelope rather than well resolved individual atomic peaks, then a molecular search, coupled with an energy minimization, is shown to be the preferable approach to solving the structure. True ab initio approaches by this method, lacking unit-cell and space-group information, can be unreliable (Chaka et al, 1996). However, the availability of electron diffraction patterns, giving cell dimensions, symmetry and intensities for comparison to a model, constrain the energy-minimization approach so that it can find a true solution to the molecular packing.…”

Study of Polymorph Prediction For L-Ascorbic Acid

“…In the past several years there has been an increasing interest in the polymorph prediction of crystal structures on the basis of molecular information. Different methods have been developed to generate possible crystal structures [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. All researches agree that there will always be a large number of possible crystal structures, as judged from their lattice energy, and nearly all are successful in finding the experimental structure.…”

Polymorph Prediction of Small Organic Molecules, Co-crystals and Salts

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