Solid materials with multiple observable phases can restructure in response to a change in temperature, fundamentally altering the materials' properties. This temperature-mediated solid transformation occurs primarily because of a difference in entropy between the two crystal forms. In this study, we examine for the first time the ability of classical point-charge molecular dynamics simulations to compute entropy and enthalpy differences between solid forms of a range of organic molecules and ultimately predict temperaturemediated restructuring events. Twelve polymorphic organic small molecule systems with known temperature-mediated transformations were modeled with the point-charge OPLS-AA potential. Relative entropies and free energies between different solid forms were estimated by computing the stability as a function of temperature from 0 K up to ambient conditions using molecular dynamics simulations. These simulations correctly found the experimental high temperature solid form to have an entropy larger than that of the low temperature form in all systems examined. The magnitude of the temperature/entropy contributions to the free energy at ambient conditions is generally larger than the change in enthalpy difference. We also find that free energy differences between polymorphs computed with a less expensive quasi-harmonic approximation are within 0.07 kcal•mol −1 at all temperatures up to 300 K in the small rigid molecules examined. However, the molecular dynamics free energies deviate from the quasi-harmonic approximation in the more flexible molecules and systems with disordered crystals by as much as 0.37 kcal•mol −1 . Finally, we demonstrate that at ambient conditions multiple lattice energy minima can convert into the same crystal ensemble due to easily kinetically accessible transitions between similar structures when thermal motions are present.

We examine the free energies of three benzene polymorphs as a function of temperature in the point-charge OPLS-AA and GROMOS54A7 potentials as well as the polarizable AMOEBA09 potential. For this system, using a polarizable Hamiltonian instead of the cheaper point-charge potentials is shown to have a significantly smaller effect on the stability at 250 K than on the lattice energy at 0 K. The benzene I polymorph is found to be the most stable crystal structure in all three potentials examined and at all temperatures examined. For each potential, we report the free energies over a range of temperatures and discuss the added value of using full free energy methods over the minimized lattice energy to determine the relative crystal stability at finite temperatures. The free energies in the polarizable Hamiltonian are efficiently calculated using samples collected in a cheaper point-charge potential. The polarizable free energies are estimated from the point-charge trajectories using Boltzmann reweighting with MBAR. The high configuration-space overlap necessary for efficient Boltzmann reweighting is achieved by designing point-charge potentials with intramolecular parameters matching those in the expensive polarizable Hamiltonian. Finally, we compare the computational cost of this indirect reweighted free energy estimate to the cost of simulating directly in the expensive polarizable Hamiltonian.

Many physical properties of small organic molecules are dependent on the current crystal packing, or polymorph, of the material, including bioavailability of pharmaceuticals, optical properties of dyes, and charge transport properties of semiconductors. Predicting the most stable crystalline form at a given temperature and pressure requires determining the crystalline form with the lowest relative Gibbs free energy. Effective computational prediction of the most stable polymorph could save significant time and effort in the design of novel molecular crystalline solids or predict their behavior under new conditions. In this study, we introduce a new approach using multistate reweighting to address the problem of determining solid-solid phase diagrams and apply this approach to the phase diagram of solid benzene. For this approach, we perform sampling at a selection of temperature and pressure states in the region of interest. We use multistate reweighting methods to determine the reduced free energy differences between T and P states within a given polymorph and validate this phase diagram using several measures. The relative stability of the polymorphs at the sampled states can be successively interpolated from these points to create the phase diagram by combining these reduced free energy differences with a reference Gibbs free energy difference between polymorphs. The method also allows for straightforward estimation of uncertainties in the phase boundary. We also find that when properly implemented, multistate reweighting for phase diagram determination scales better with the size of the system than previously estimated.

Two new calcium coordination polymers, [Ca 3 (btc) 2 (H 2 O) 12 ](1) and [Ca 2 (btc)(pzc)(H 2 O) 3 ] (2) (btc = benzene-1,3,5-tricarboxylate, pzc = pyrazine-2-carboxylate), have been synthesized using the hydro/ solvothermal method and have been characterized using X-ray diffraction, IR, UV−vis, thermogravimetric analysis, and fluorescence analysis. The structure of compound 1 is a three-dimensional framework consisting of helical chains of calcium coordination polymers, while that of compound 2 is a double layered network in which the inorganic zigzag chains of calcium coordination polyhedra are linked by organic ligands. Both compounds show blue fluorescence when excited with UV light. Density functional theory calculations on electronic absorption spectra of organic ligands and calcium coordination polymers are discussed.

We report a comprehensive study of the crystal structure of (Ga(1-x)Znx)(N(1-x)Ox) solid solution nanoparticles by means of neutron and synchrotron X-ray scattering. In our study, we used four different types of (Ga(1-x)Znx)(N(1-x)Ox) nanoparticles, with diameters of 10-27 nm and x = 0.075-0.51, which show energy band gaps from 2.21 to 2.61 eV. Rietveld analysis of the neutron diffraction data revealed that the average crystal structure is hexagonal wurtzite (space group P63mc) for the larger nanoparticles, while the crystal structure of smaller nanoparticles is disordered hexagonal. Pair-distribution-function analysis found that the intermediate crystal structure retains a "motif" of the average one; however, the local structure is more disordered. The implications of disorder on the reduced energy band gap are discussed.