Neil L. Allan

The embedded many-body expansion for energetics of molecular crystals

Reliable prediction of molecular crystal energetics is a vital goal for computational chemistry. Here we show that accurate results can be obtained from a monomer-based many-body expansion truncated at the two-body level, with the monomer and dimer calculations suitably embedded in a model of the crystalline environment. By including the two dominant effects--electrostatics and exchange-repulsion--we are able to capture the important nonadditive terms in the energy, and approach very closely results from full periodic second-order Møller-Plesset calculations. The advantage of the current scheme is that extension to coupled-cluster and explicitly correlated F12 methods is completely straightforward. We demonstrate the approach through calculations on carbon dioxide, hydrogen fluoride, and ice XIh and XIc. In accord with previous studies, we find these two ice polymorphs to be very close in energy, with our periodic coupled-cluster single double triple-F12 calculation giving the hexagonal structure more stable by around 0.3 kJ mol(-1).

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.

A novel approach to molecular similarity

SummaryWe review briefly the general problem of assessing the similarity between one molecule and another. We propose a novel approach to the quantitative estimation of the similarity of two electron distributions. The procedure is based on momentum space concepts, and avoids many of the difficulties associated with the usual position space definitions. Results are presented for the model systems CH3CH2CH3, CH3OCH3, CH3SCH3, H2O and H2S.

Isovalent trace element partitioning between minerals and melts - a computer simulation study

Abstract We present a new approach for the rationalisation of trace element partitioning between silicate melts and minerals, which is not based on the empirical, parameterised continuum models in common use. We calculate the energetics of ion substitution using atomistic simulation techniques, which include an explicit evaluation of the relaxation energy (strain energy) contribution to this process. Solution energies are estimated for isovalent impurities in CaO, diopside, orthoenstatite, and forsterite. These show a parabolic dependence on ionic radius, similar to the variation of mineral-melt partition coefficients with ionic radius. The success of the empirical models, which often include only the strain energy, appear to have been due to the partial cancellation of energy terms, and to the empirical fitting of the parameters included in these models. Our approach can be readily extended to aliovalent substitution.

Ab initio Hartree-Fock calculations of CaO, VO, MnO and NiO

Abstract Ab initio Hartree-Fock calculations are reported of the electronic structures of CaO, VO, MnO and NiO. They are found to be essentially ionic in nature with ground states that are insulating, and, in the case of VO, MnO and NiO, high-spin antiferromagnetic. Calculated lattice parameters, binding energies and bulk moduli compare in accuracy with those for closed-shell oxides such as Li2O, MgO and Al2O3. Differences are found in the upper part of the valence band between VO and MnO and NiO. In VO it is predominantly V(3d) in character, whereas in both MnO and NiO it is O(2p). In the case of NiO this seems to be in accord with oxygen K-edge data for LixNi1−xO.

Direct evidence of O(p) holes in Li-doped NiO from Hartree-Fock calculations

Abstract First-principles periodic Hartree-Fock calculations of the ground state electron distribution and empty oxygen p states in Li 0.125 Ni 0.875 O and Li 0.25 Ni 0.75 O are reported which provide direct evidence of oxygen p holes in Li-doped NiO. Calculated changes in the densities of empty oxygen p states are in good agreement with oxygen K -edge spectra. The empty states of the Li-doped materials provide a theoretical value of the band gap in NiO which, unlike previous estimates, is reasonably close to the observed value of 3.7 eV.

Displacement cascades in Gd2Ti2O7 and Gd2Zr2O7: a molecular dynamics study

We report molecular dynamics simulations of the production of radiation cascades in two pyrochlore compounds that have been proposed as possible materials for high level radioactive waste storage. There are clear differences between the two systems that support the results of recent high energy ion bombardment experiments, in which pyrochlores were increasingly radiation resistant with increasing Zr content.

Ab initio calculation of phase diagrams of ceramics and minerals

A range of methods, based on Monte Carlo and lattice dynamics simulations, are presented for the calculation of the thermodynamic properties of solid solutions and phase diagrams. These include Monte Carlo simulations with the explicit interchange of cations, the use of the semigrand-canonical ensemble and configurational bias techniques, hybrid Monte Carlo/molecular dynamics, and a new configurational lattice dynamics technique. It is crucial to take account of relaxation of the local atomic environment and vibrational effects. Examples studied are (i) the enthalpy and entropy of mixing, the phase diagram and the spinodal of MnO/MgO. The available experimental data disagree widely for this system; (ii) the enthalpy of mixing of CaO/MgO, where the size mismatch between the cations is considerably larger than in (i); (iii) the postulated high-pressure orthorhombic to cubic phase transition in (Mg,Mn)SiO3 perovskite, where we show that impurity cations can have a much larger effect than that expected from a mean-field treatment or linear interpolation between end-member compounds.

Simulations of chemical vapor deposition diamond film growth using a kinetic Monte Carlo model

A one-dimensional kinetic Monte Carlo model has been developed to simulate the chemical vapor deposition (CVD) of a diamond (100) surface. The model considers adsorption, etching/desorption, lattice incorporation, and surface migration along and across the dimer rows. The reaction rates for these processes are re-evaluated in detail and their effect upon the predicted growth rates and morphology are described. We find that for standard CVD diamond conditions, etching of sp3 carbon species from the growing surface is negligible. Surface migration occurs rapidly, but is mostly limited to CH2 species oscillating back and forth between two adjacent radical sites. Despite the average number of migration hops being in the thousands, the average surface diffusion length for a surface species—before it either adds to the diamond lattice or is removed back to the gas phase—is <2 sites. β-scission helps to smooth the surface, but is only a relatively minor process removing <2% of adsorbed species. At low substrate ...

Oxygen ion migration in La2CuO4

Abstract Calculated migration energies for oxygen vacancies and interstitial oxide ions in tetragonal La2CuO4 are reported. These are low and similar in magnitude to those for oxygen vacancy migration in binary oxide systems such as zirconia. An important difference between zirconia and these ternary cuprates, however, is that calculated association energies between divalent cation impurities and oxygen vacancies are much smaller in the latter.