The electron affinity ͑EA͒ of various terminations of diamond surfaces has been calculated by the ab initio pseudopotential method. The bare, reconstructed ͑100͒ and ͑111͒ surfaces are found to have positive EA's of 0.5 and 0.35 eV, respectively. The hydrogen-terminated surfaces 1ϫ1(100):2H, 2ϫ1 (100):H, and ͑111͒:H have sizable negative EA's of order Ϫ2.4, Ϫ2.0, and Ϫ2.0 eV, respectively. A symmetrical canting was found to be the most stable geometry for the 1ϫ1(100):2H surface. The oxygen-terminated surfaces have positive affinities of ϩ2.6 eV for the more stable ether configuration, while the OH termination has a negative EA. The various values can be understood in terms of the surface dipole of the terminating bond.
The c2x code fills two distinct roles. Its first role is in acting as a converter between the binary format .check files from the widely-used Castep[1] electronic structure code and various visualisation programs. Its second role is to manipulate and analyse the input and output files from a variety of electronic structure codes, including Castep, Onetep and Vasp, as well as the widelyused 'Gaussian cube' file format. Analysis includes symmetry analysis, and manipulation arbitrary cell transformations. It continues to be under development, with growing functionality, and is written in a form which would make it easy to extend it to working directly with files from other electronic structure codes.Data which c2x is capable of extracting from Castep's binary checkpoint files include charge densities, spin densities, wavefunctions, relaxed atomic positions, forces, the Fermi level, the total energy, and symmetry operations. It can recreate .cell input files from checkpoint files. Volumetric data can be output in formats usable by many common visualisation programs, and c2x will itself calculate integrals, expand data into supercells, and interpolate data via combinations of Fourier and trilinear interpolation. It can extract data along arbitrary lines (such as lines between atoms) as 1D output.C2x is able to convert between several common formats for describing molecules and crystals, including the .cell format of Castep. It can construct supercells, reduce cells to their primitive form, and add specified k-point meshes. It uses the spglib library[2] to report symmetry information, which it can add to .cell files.C2x is a command-line utility, so is readily included in scripts. It is available under the GPL and can be obtained from http://www.c2x.org.uk. It is believed to be the only open-source code which can read Castep's .check files, so it will have utility in other projects.
Silicon carbide often grows in the cubic phase under conditions where this is not the most stable phase. Ab initio calculations are presented which determine the energy of a stacking reversal at the ( 0001) and (000 1) surfaces of silicon carbide, and thus whether the cubic form is preferred in the vicinity of a free surface. In these calculations the surfaces are not reconstructed but hydrogen terminated, and care is taken to eliminate the spurious dipole-dipole interaction caused by the imposition of periodic boundary conditions, and also in estimating the amount of residual ionic relaxation. These calculations do show a clear distinction of 13 meV per surface pair between the silicon and carbon surfaces, although the results are not in complete agreement with experiment.
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