High-density Crystals Research Articles

Phase behavior of hard cylinders.

Using isobaric Monte Carlo simulations, we map out the entire phase diagram of a system of hard cylindrical particles of length (L) and diameter (D) using an improved algorithm to identify the overlap condition between two cylinders. Both the prolate L/D > 1 and the oblate L/D < 1 phase diagrams are reported with no solution of continuity. In the prolate L/D > 1 case, we find intermediate nematic N and smectic SmA phases in addition to a low density isotropic I and a high density crystal X phase with I-N-SmA and I-SmA-X triple points. An apparent columnar phase C is shown to be metastable, as in the case of spherocylinders. In the oblate L/D < 1 case, we find stable intermediate cubatic (Cub), nematic (N), and columnar (C) phases with I-N-Cub, N-Cub-C, and I-Cub-C triple points. Comparison with previous numerical and analytical studies is discussed. The present study, accounting for the explicit cylindrical shape, paves the way to more sophisticated models with important biological applications, such as viruses and nucleosomes.

Crystal packing of ?strained? organic molecules

Comparison of the energy of nonbonding interactions in the molecules of a series of conformers and isomers showed that a looser molecular crystal packing corresponds to the conformer or isomer with greater Unb. Analysis of the distribution of the packing coefficients (Kp) for 159 medium and high density organic crystalline structures indicates that the fraction of structures with low Kp is greater for the high density crystals. The minimal Kp (0.612) is close to the value predicted by Kitaigorodskii in the dense packing theory (0.60). A tendency was noted to decreasing Kp with increasing molecular density.