Beate Paulus

On the accuracy of correlation-energy expansions in terms of local increments

The incremental scheme for obtaining the energetic properties of extended systems from wave-function-based ab initio calculations of small (embedded) building blocks, which has been applied to a variety of van der Waals-bound, ionic, and covalent solids in the past few years, is critically reviewed. Its accuracy is assessed by means of model calculations for finite systems, and the prospects for applying it to delocalized systems are given.

Cohesive energies of cubic III-V semiconductors

Cohesive energies for twelve cubic III-V semiconductors with zinc-blende structure have been determined using an ab initio scheme. Correlation contributions, in particular, have been evaluated using the coupled-cluster approach with single and double excitations. This was done by means of increments obtained for localized bond orbitals and for pairs and triples of such bonds. Combining these results with corresponding Hartree-Fock data, we recover about 92% of the experimental cohesive energies. textcopyright{} 1996 The American Physical Society.

INFLUENCE OF ELECTRON CORRELATIONS ON GROUND-STATE PROPERTIES OF III-V SEMICONDUCTORS

Lattice constants and bulk moduli of eleven cubic III-V semiconductors are calculated using an ab initio scheme. Correlation contributions of the valence electrons, in particular, are determined using increments for localized bonds and for pairs and triples of such bonds; individual increments, in turn, are evaluated using the coupled cluster approach with single and double excitations. Core-valence correlation is taken into account by means of a core polarization potential. Combining the results at the correlated level with corresponding Hartree-Fock data, we obtain lattice constants that agree with experiment within an average error of -0.2%; bulk moduli are accurate to +4%. We discuss in detail the influence of the various correlation contributions on lattice constants and bulk moduli.

Electron correlations for ground-state properties of group-IV semiconductors

Valence energies for crystalline C, Si, Ge, and Sn with diamond structure have been determined using an ab-initio approach based on information from cluster calculations. Correlation contributions, in particular, have been evaluated in the coupled electron pair approximation (CEPA), by means of increments obtained for localized bond orbitals and for pairs and triples of such bonds. Combining these results with corresponding Hartree-Fock (HF) data, we recover about 95 % of the experimental cohesive energies. Lattice constants are overestimated at the HF level by about 1.5 %; correlation effects reduce these deviations to values which are within the error bounds of this method. A similar behavior is found for the bulk modulus: the HF values which are significantly too high are reduced by correlation effects to about 97 % of the experimental values.

Application of the method of increments to the adsorption of CO on the CeO2(110) surface

We have combined an embedded-cluster model with an extension of the method of increments to treat the adsorption of molecules on a surface. In this way we are able to investigate the physisorption of CO on CeO(2)(110) at the MP2, MP4(SDTQ), and CCSD(T) levels with only moderate computational costs. We find that, at the CCSD(T) level, 25% of the adsorption energy originates from electron correlation. The interactions of the CO molecule with its five nearest cerium and oxygen neighbors in the surface layer make the largest contributions to the electron correlation. Approximately 97% of the adsorption-induced electron correlation energy part of the adsorption energy is recovered by the method of increments (in our chosen expansion), at the MP2 level.

Convergence of the ab initio many-body expansion for the cohesive energy of solid mercury

A many-body expansion for mercury clusters of the form E = sum_{i<j}Delta epsilon_{ij} + sum_{i<j<k}Delta epsilon_{ijk} + ... quad, does not converge smoothly with increasing cluster size towards the solid state. Even for smaller cluster sizes (up to n=6), where van der Waals forces still dominate, one observes bad convergence behaviour. For solid mercury the convergence of the many-body expansion can dramatically be improved by an incremental procedure within an embedded cluster approach. Here one adds the coupled cluster many-body electron correlation contributions of the embedded cluster to the bulk HF energy. In this way we obtain a cohesive energy (not corrected for zero-point vibration) of 0.79 eV in perfect agreement with the experimental value.

Ground-state properties of rutile: Electron-correlation effects

Electron-correlation effects on cohesive energy, lattice constant, and bulk compressibility of rutile are calculated using an ab initio scheme. A competition between the two groups of partially covalent Ti-O bonds is the reason that the correlation energy does not change linearly with deviations from the equilibrium geometry, but is dominated by quadratic terms instead. As a consequence, the Hartree-Fock lattice constants are close to the experimental ones, while the compressibility is strongly renormalized by electronic correlations.

A correlated ab initio treatment of the zinc-blende wurtzite polytypism of SiC and III - V nitrides

Ground-state properties of SiC, AlN, GaN and InN in the zinc-blende and wurtzite structures are determined using an ab initio scheme. For the self-consistent-field part of the calculations, the Hartree - Fock program CRYSTAL has been used. Correlation contributions are evaluated using the coupled-cluster approach with single and double excitations. This is done by means of increments derived for localized bond orbitals and for pairs and triples of such bonds. At the Hartree - Fock level, it turns out that for SiC the zinc-blende structure is more stable although the very small energy difference from the wurtzite structure is an indication of the experimentally observed polytypism. For the III - V nitrides the wurtzite structure is found to be significantly more stable than the zinc-blende structure. Electron correlations do not change the Hartree - Fock ground-state structures, but energy differences are enlarged by up to 40%. While the Hartree - Fock lattice parameters agree well with experiment, the Hartree - Fock cohesive energies reach only 45% to 70% of the experimental values. Including electron correlations, we recover for all compounds about 92% of the experimental cohesive energies.

Correlation calculations for the reconstruction of the Si (100) surface

Abstract Ab initio multi-reference configuration interaction calculations are performed for the Si(100) surface using a cluster approach. The convergence with respect to the cluster size is checked and the final results are taken from a Si32H28 cluster which models two dimers and six bulk layers. We find for the ideal as well as for the p(1×2) reconstruction a singlet ground state consisting of several configurations. The energy gain due to forming the symmetric dimer in the p(1×2) structure is 1.75xa0eV, the bond length of the dimer is 2.35xa0A, which is very close to the bulk value. In contradiction to the local density approximation results and in agreement with previous correlation calculations we do not find an asymmetric p(1×2) structure.

Correlated ab initio calculations for ground-state properties of II-VI semiconductors

Correlated ab-initio ground-state calculations, using relativistic energy-consistent pseudopotentials, are performed for six II-VI semiconductors. Valence (