Andrew L. Rohl

The general utility lattice program (GULP)

The General Utility Lattice Program (GULP) has been extended to include the ability to simulate polymers and surfaces, as well as adding many other new features, and the current status of the program is fully documented. Both the background theory is described, as well as providing a concise review of some of the previous applications in order to demonstrate the range of its use. Examples are presented of work performed using the new compatibilities of the software, including the calculation of Born effective charges, mechanical properties as a function of applied pressure, calculation of frequency-dependent dielectric data, surface reconstructions of calcite and the performance of a linear-scaling algorithm for bond-order potentials.

MARVIN : A NEW COMPUTER CODE FOR STUDYING SURFACES AND INTERFACES AND ITS APPLICATION TO CALCULATING THE CRYSTAL MORPHOLOGIES OF CORUNDUM AND ZIRCON

A new computer code (MARVIN), employing two-dimensional (2D) periodic boundary conditions, has been developed for the simulation of surfaces and interfaces. The models and methodologies incorporated within the program are discussed. The utility of the program in calculating crystal morphologies is explored using α-Al2O3 and zircon as examples. The important aspects of these calculations are that they include the use of covalent-type force fields in the latter potential model and that the effects of surface relaxation on the growth morphology are calculated for the first time. It is demonstrated that relaxation has a much larger effect on the equilibrium morphology than the growth morphology, but it can still be significant on the latter. A previously derived relationship between the growth and equilibrium morphologies is shown not to hold for relaxed systems. The growth morphologies are found to be in better agreement with experiment than the equilibrium morphologies since the latter overestimates the importance of high-index faces, especially after relaxation. Finally, the calculated surface relaxation for the basal plane of α-Al2O3 is found to be in complete agreement with Hartree–Fock ab initio calculations, verifying that the bulk potentials transfer to this surface.

GDIS: a visualization program for molecular and periodic systems

Abstract GDIS is a freely available chemical visualization program for displaying molecules, periodic structures, and crystal morphologies. A key feature of the package is the mechanism for constructing and manipulating arbitrary crystal surfaces. This enables GDIS to offer a powerful tool set for computing surface and interfacial properties and predicting crystal morphologies. Also included are modules for computing powder diffraction patterns, generating molecular surfaces, and analyzing dynamics trajectories. Further, the program may be used as a graphical interface to simplify the construction of input files for the command line codes: GULP, GAMESS(US), and SIESTA.

Evidence from surface phonons for the (2 × 1) reconstruction of the (101̅4) surface of calcite from computer simulation

Abstract A new force field for modeling calcium carbonate has been derived that corrects deficiencies of previous models. The model correctly reproduces the structure of the gas phase species, as predicted from ab initio calculations, as well as the bulk structure and properties of calcite and aragonite. With this new model, a (2 × 1) reconstruction is predicted to occur for the dominant (101̄4) surface of calcite, involving rotation of half of the surface carbonate anions. This reconstruction matches the results of low energy electron diffraction measured in vacuo and provides the first independent verification of this observation, as well as yielding the atomic detail of the nature of the reconstruction. While there is only a small exothermic energy associated with the formation of the supercell, the presence of an imaginary phonon mode at (½0) in the two-dimensional Brillouin zone for a single surface cell verifies the existence of the reconstruction.

Structure, stability and morphology of stoichiometric ceria crystallites

The atomistic structure of an extensive set of ceria surfaces are predicted using four different inter-atomic potential models. The dependence of the results on the parameters is discussed in detail. For example, we find that while absolute surface energies vary considerably, relative energies do not. As such, an octahedral crystallite morphology can be predicted with confidence. However, for one model the predicted surface ion relaxations are large and very different compared to those of the other three models. This is due, in part, to the difficulties of applying shell model parameters derived from bulk calculations to surface studies.

Calculated bulk and surface properties of sulfates

Atomistic simulation techniques are used to model a range of sulfates. Two widely different sets of potentials have been developed. The first is based on shell model, electron-gas potentials; the second is a rigid ion model which treats inter- and intra-molecular forces differently. The success of the potential models has been demonstrated by comparing calculated and experimental lattice parameters and elastic constants. The structures and energetics of surfaces of barite (BaSO4) are examined in detail, allowing for the effects of surface relaxation. The two lowest-energy surfaces are {001} and {210} which dominate the calculated equilibrium morphology.

Molecular modeling of water adsorption on hematite

This paper describes the results of modeling the surface hydration configurations formed when different planes of the hematite crystal were exposed to water using empirically derived potentials able to replicate the hematite, goethite and lepidocrocite structures to within 2% of their measured values. The planes chosen were the {111}, {011} and {210} planes expressed in rhombohedral coordinates. It was found that of all the surfaces studied there was a preference for hydration on the O-terminated basal {111} plane. This plane had the lowest hydrated surface energy and it was also the most stabilised by reaction with water. The Fe-terminated {111} plane was found to be unstable in the presence of excess water (67% coverage). The surface iron atoms relax away from the simulation cell to leave the O-terminated hydrated layer behind. Chemisorption may be energetically feasible at low surface coverages (<67% coverage). The {011} plane of hematite showed a preference for 100% water coverage (full coordination of the surface iron atoms). The surface energy of adsorbing water on this plane was lower than for the {210} plane particularly at high water coverages. The {210} plane was not stabilised by reaction with water at any coverage. The surfaces underwent relaxations depending on the water coverage. Large relaxations were observed at lower coverages for the {011} plane while the largest relaxations were observed at higher coverages on the {210} plane.

Atomistic theory of the interaction between AFM tips and ionic surfaces

We discuss the results of atomistic calculations of the interaction between three different types of AFM tips composed of MgO and SiO2, and perfect and defective (001) surfaces of LiF, NaCl and CaO.

Twisted aspirin crystals

Banded spherulites of aspirin have been crystallized from the melt in the presence of salicylic acid either generated from aspirin decomposition or added deliberately (2.6-35.9 mol %). Scanning electron microscopy, X-ray diffraction analysis, and optical polarimetry show that the spherulites are composed of helicoidal crystallites twisted along the <010> growth directions. Mueller matrix imaging reveals radial oscillations in not only linear birefringence, but also circular birefringence, whose origin is explained through slight (∼1.3°) but systematic splaying of individual lamellae in the film. Strain associated with the replacement of aspirin molecules by salicylic acid molecules in the crystal structure is computed to be large enough to work as the driving force for the twisting of crystallites.

Computer prediction of crystal morphology

Control of crystal shape is of great importance to several contemporary industries and thus there is a need to understand the fundamental processes determining crystal morphology. Computer simulations have been widely used in an attempt to predict the morphology of crystals in real systems under a variety of conditions and this review demonstrates that great strides have been made in this area over the last 18 months.