Soft Condensed Phases
Molecular crystals are bound by intermolecular (van der Waals) forces, and knowledge of such force fields should be sufficient to predict crystal structures. In principle, accurate force fields can be obtained using electronic structure methods, but this has not yet been achievable in practice. Thus, theoretical investigations of crystal structures typically rely on empirical force fields that are parametrized using experimental information.
Unfortunately, the predictive capability of such fields is limited, since a given field can describe well only the system used for its parametrization and thus is often not transferable even to polymorphs of this system. As a result, prediction of crystal structures has been considered an impossible task. In 1988, J. Maddox published a provocative op-ed in Nature stating that "one of the continuing scandals" is that computational scientists are not able to predict crystal structures from molecular structures---the issue has eluded scientists for more than 50 years and as emphasized by the low success rate of crystal structure predictions in the blind tests conducted
by the Cambridge Crystallographic Data Center.
One of the key issues in predicting crystal structures is the accuracy of the force fields. This accuracy is also critical for calculations of lattice energies at experimental crystal structures. The force fields can be computed ab initio using wave-function (WF) based methods, but until recently the accuracy achievable for molecules containing more than a few atoms was far from quantitative and was insufficient for determination of crystal structures. One might have hoped that the problem could be resolved by the development of density functional theory (DFT), which can be applied to systems containing hundreds of atoms. Unfortunately, conventional DFT methods fail badly in describing intermolecular interactions for which dispersion is the dominant component; such systems include molecular organic crystals.
Recently, Szalewicz group has proposed a method which combines symmetry-adapted perturbation theory (SAPT) of intermolecular interactions with the Kohn-Sham DFT representation of monomers. SAPT is a perturbational approach starting from isolated Hartree-Fock or Kohn-Sham monomers and imposing the correct permutational symmetry on perturbed wave functions. In the latter case, some time consuming terms describing intramonomer electron correlation can be omitted, resulting in a very low-cost method which gives predictions as accurate as those of high-level WF-based methods.
SAPT(DFT) has been so far applied mainly to dimers. In recent studies it has also been utilized to predict structures and lattice energies of extended objects: molecular crystals. An example of such analysis is crystal of cyclotrimethylene trinitramine (RDX) shown in the Figure above. We have demonstrated that the SAPT(DFT) method is capable of producing force fields for interactions of relatively large organic molecules that enable reliable predictions of crystal structures of these compounds.
The predictions can be done entirely from first principles, eliminating reliance on empirical force fields and enabling treatment of compounds for which experimental data are unavailable. This method is expected to find broad applications in crystal design, in particular, to screening novel materials and drug candidates, screening molecules for cocrystallization, and identification of low-energy polymorphs of pharmaceutical compounds
- molecular simulations of liquids and solids
- predictions of structure of molecular crystals.
R. Podeszwa, B. M. Rice, and K. Szalewicz, Predicting structure of molecular crystals from first principles, Phys. Rev. Lett. 101, 115503 (2008). [PDF]
A. J. Misquitta, R. Podeszwa, B. Jeziorski, and K. Szalewicz, Intermolecular potentials based on symmetry-adapted perturbation theory including dispersion energies from time-dependent density functional calculations, J. Chem. Phys. 123, 214103 (2005). [URL]