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The phase diagram of cyclopentane has been studied by powder neutron diffraction, providing diffraction patterns for phases I, II, and III, over a range of temperatures and pressures. The putative phase IV was not observed. The structure of the ordered phase III has been solved by single-crystal diffraction. Computational modeling reveals that there are many equienergetic ordered structures for cyclopentane within a small energy range. Molecular dynamics simulations reproduce the structures and diffraction patterns for phases I and III and also show an intermediate disordered phase, which is used to interpret phase II.

Rigid-body, k = 0 phonon frequencies have been calculated within the crystal structure modeling program DMAREL, enabling the use of anisotropic atom−atom model potentials. Five organic crystals (hexamethylenetetramine, naphthalene, pyrazine, imidazole, and α-glycine) were chosen to sample a range of intermolecular interactions for determining the sensitivity of the calculated frequencies to changes in the empirical repulsion-dispersion parameters and the electrostatic model. A carefully parameterized simple exp-6 model can describe vibrations in simple van der Waals crystals and some hydrogen bonded crystals reasonably well. However, for weaker polar interactions, an accurate model of the electrostatics is needed. Bending of weak polar interactions and shearing of close contacts with delocalized π-systems are particularly sensitive to the description of electrostatic interactions. Point charge models generally underestimate the resistance to deforming hydrogen bonds, and a distributed multipole model stab...

A series of nonempirical intermolecular potentials has been developed for the cyanuric chloride dimer, using the overlap model to determine the anisotropy of the repulsive wall around each atom. Calibration against intermolecular perturbation theory calculations enables the penetration and charge-transfer energy to be explicitly included with the exchange-repulsion to give a simple repulsion model in an anisotropic atom−atom form. These model repulsion potentials are used in conjunction with an atomic multipole electrostatic model and an atom−atom dispersion model to give nonempirical potential models, which are tested for their ability to reproduce the crystal structure of cyanuric chloride. The best nonempirical potential is successfully used to construct a simpler transferable model for closely related azaaromatic chlorides. The nonempirical potential reproduces the experimental space group of cyanuric chloride, unlike some empirically fitted repulsion−dispersion potentials. This first nonempirical rep...

Elastic constants of a set of molecular organic crystals have been calculated within the crystal modeling program DMAREL, which was developed to allow the use of highly accurate, anisotropic atom−atom potentials. A set of six molecular crystals (durene, m-dinitrobenzene, the β form of resorcinol, pentaerythritol, urea, and hexamethylenetetramine) has been chosen to sample a range of intermolecular interactions and crystal symmetries. The sensitivity of such calculations to variations in empirical repulsion−dispersion parameters and the electrostatic model is determined and discussed relative to the other errors in the theoretical model and typical experimental uncertainties. We find that model potentials whose functional form has been simplified often reproduce crystal structures and lattice energies adequately but perform poorly when used to calculate elastic constants. This is because more theoretically justified potentials are required to satisfactorily model the curvature at the equilibrium geometries...

By use of inelastic incoherent neutron scattering techniques on ISIS at Rutherford-Appleton Laboratory, neutron spectra for a large range of recoverable high-pressure phases of ice have been measured. In order to understand the physical significance of the measured spectra and reproduce the experimental data, we have proposed a model, that requires two strengths of hydrogen bonding. In this paper, we describe some aspects of this model in detail and compare it with the existing potential models.

Since the observation of two intermolecular translational optic peaks at 28 and 37 meV using inelastic neutron scattering techniques, there has been a renewed effort to reproduce phonon density of states with these peaks by computer simulation. In this paper, we demonstrate our attempt to calculate the measured inelastic neutron spectrum using molecular dynamic techniques for a number of potentials, such as TIP4P, MCY, and KKY potentials. The results show that none of them can reproduce the double peaks seen in the measured spectrum.

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...