Search

Free energies of crystals computed using a center of mass constraint require a finite-size correction, as shown in previous work by Polson et al. [J. Chem. Phys. 112, 5339–5342 (2000)]. Their reference system is an Einstein crystal with equal spring constants. In this paper, we extend the work of Polson et al. [J. Chem. Phys. 112, 5339–5342 (2000)] to the case of differing spring constants. The generalization is convenient for constraining the center of mass in crystals with atoms of differing masses, and it helps to optimize the free energy calculations. To test the theory, we compare the free energies of LiI and NaCl crystals from calculations with differing spring constants to those computed using equal spring constants. Using these center of mass finite size corrections, we compute the true free energies of these crystals for different system sizes to eliminate the intrinsic finite-size effects. These calculations help demonstrate the size of these finite-size corrections relative to other contributions to the absolute free energy of the crystals. [ABSTRACT FROM AUTHOR]

Solid–fluid phase equilibria are difficult to predict in simulations because bound degrees of freedom in the crystal phase must be converted to free translations and rotations in the fluid phase. Here, we avoid the solid-to-fluid transformation step by starting with chemical potentials for two reference systems, one for the fluid phase and one for the solid phase. For the solid, we start from the Einstein crystal and transform to the fully interacting molecular crystal. For the fluid phase, we introduce a new reference system, the "centroid," and then transform to gas phase molecules. We illustrate the new calculations by predicting the sublimation vapor pressure of succinic acid in the temperature range of 300 K–350 K. [ABSTRACT FROM AUTHOR]

This work uses a dual absolute chemical potential route with a centroid reference for the dissolved molecules and an Einstein crystal reference for the solid. We use these calculations to predict the vapour pressure, solubility limit, and chemical potential driving forces for crystallization of napthalene (large and rigid) and succinic acid (floppy and anharmonic). We examine each source of error in the free energy calculations and demonstrate that the chemical potentials can be computed with an overall precision of ca. 0.05 kcal/mol, leading to solubility predictions with precision of ca. 10%. The precise calculations should facilitate studies of nucleation kinetics, growth kinetics, and efforts to develop accurate force fields. [ABSTRACT FROM AUTHOR]

One-dimensional (1D) nucleation is important in crystal growth, amyloid fibril formation, and supramolecular polymerization. The nucleation rate can be readily calculated, but there is no consensus on the critical length. In this work, we employ a splitting probability to derive an analytical expression for the critical length, which corresponds to a 50% survival probability. For a 1D nucleus on a crystal step, this critical length depends on the supersaturation (driving force) and is nearly independent of the kink (surface) energy. [ABSTRACT FROM AUTHOR]

The surface free energy of a droplet is approximately γ(R)=γ(∞)(1−2δ/R), with R being the droplet radius and δ being the Tolman length. Here we use the mitosis method to compute δ=−0.56±0.1Å at 300 K for mW water, indicating that γ(R) increases as the droplet size decreases. The computed Tolman length agrees quite well with a previous study of TIP4P/2005 water. We also decompose the size-dependent surface free energy into energetic and entropic contributions for the mW and TIP4P/2005 force fields. Despite having similar Tolman lengths, the energy-entropy decompositions are very different for the two force fields. We discuss critical assumptions which lead to these findings and their relation to experiments on the nucleation of water droplets. We also discuss surface broken bonds and structural correlations as possible explanations for the energetic and entropic contributions. [ABSTRACT FROM AUTHOR]

Particle production and solids processing are essential components of the contemporary process industries. Crystalline solids represent a large and important segment of this manufacturing sector. Chemical engineers, especially in the United States, have historically abandoned this subject, leaving it to pharmacists, physical chemists, material scientists, geologists, and other disciplines. In fact, crystalline solids have become the lost phase of the chemical engineering curriculum. Both the subject and the community are large; it is easy to assemble over 300 delegates at crystallization conferences, and there are many journals and books devoted to crystals. In my view this is a missed opportunity for chemical engineers, and also a missed opportunity for the field of crystal engineering. Crystal engineering has evolved into a mostly descriptive field (the materials science focus) with only a small component concerned with quantitative aspects. Chemical engineers are uniquely positioned to make major quantitative contributions to crystal engineering like they did for industrial chemistry and fluid-phase processes during the twentieth century. Moreover, crystalline solid products offer an excellent platform for advancing the field of integrated product and process design. Changes to the chemical engineering curriculum are recommended in order to provide graduates with the education necessary to compete for jobs in this manufacturing sector.