Equilibrium Geometric Structures Research Articles

Total energy minimization for surfaces of covalent semiconductors C, Si, Ge, and α-Sn: I. (111)2×1 surfaces

A semi-empirical tight-binding approach to the total energy of a surface system is presented. The resulting energy functional includes the electron-electron and ion-ion interaction in a reasonable way. It incorporates a self-consistent treatment of energies and charge transfers and is therefore capable of handling complex surface reconstruction geometries. The energy gain due to reconstruction and the resulting equilibrium surface geometry follow from the minimization of the total energy functional with respect to the atomic coordinates. The method is applied to the (111)2×1 surfaces of diamond, silicon, germanium, and α-Sn. Three reconstruction models are studied: the Haneman buckling model, the Pandey π-bonded chain geometry, and the Chadi molecule structure. Among the models suggested for the 2×1 reconstructed (111) surfaces the π-bonded chain model has to be favoured from the energetical point of view. The energy minimization in the framework of the buckling model only yields the ideal bulk-terminated surface structure. The molecule reconstruction does not give rise to any positive energy gain. For the π-bonded chain model the deepest energy is found for slightly buckled but undimerized chains. Chemical trends are discussed. The minimum-energy equilibrium geometrical structures obtained for this model are compared with atomic configurations extracted from other total energy calculations, medium-energy ion scattering and dynamical LEED. Overall agreement is stated. However there are also characteristic differences with respect to the magnitude of the tilts in the individual atomic layers, the relaxation of these layers and the strain resulting in these geometries. The atomic structures obtained experimentally are tested by means of the presented total energy functional.

Defect Studies in Silicon Dioxide by Local Density Approximation Total Energy Methods

AbstractThe oxygen vacancy in silicon dioxide is currently believed to explain one of the most important defects observed in this material, the E1 ' center. To get a better understanding of the oxygen vacancy, we have investigated the equilibrium geometric structures, total energies, and charge densities of several charge states of the defect in alpha cristobalite. We also present some preliminary results for alpha quartz.

Crystalline As2Se3: Electronic and geometric structure.

Realistic ab initio total-energy calculations are performed to investigate the electronic and geometric structure of crystalline ${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$. Results include the following: total energies for various distortions, the equilibrium geometric structure, intralayer and interlayer cohesive energies, charge densities, the density of states, the band structure, rigid-layer phonon frequencies, and effective masses. Specific attention is focused on studying the nature of bonding within and between layers, as well as elucidating the nature of the electron and hole wave functions in the vicinity of the fundamental gap. In particular it is found that the interactions between the layers are crucial in determining the properties of the wave functions at the band edges. Moreover, the electron and hole masses are predicted to exhibit a large and unusual anisotropy.

Equilibrium structure of the molecular cation radical of ethyl acetate in the gas phase: Mechanism of formation of rearrangement ions

The equilibrium (spatial and geometric) structures of the molecular cation radical of ethyl acetate have been studied using the SCF MO LCAO method in t The molecular ion in the equilibrium state is shown to have a five-membered cyclic structure. Vertical and adiabatic ionization energies have been calc

SCF-CI studies of the equilibrium structure and the proton transfer barrier H3O 2 ?

Large-scale configuration interaction (CI) calculations have been performed in order to study the effect of the correlation energy on the equilibrium geometrical structure, the stability, and on the energy barrier of the proton transfer reaction in the hydrogen bonded system HO− · HOH. An extended Gaussian basis set including polarization functions on each nuclear centre has been employed to approximate the molecular Orbitals. All possible single and double replacements resulting from a single determinant Hartree-Fock reference state have been taken into account in the CI wavefunction. Compared to the SCF results the equilibrium oxygen/oxygen distance has been obtained from the CI calculations to be smaller by about 0.08 A and the correlation energy has been found to stabilize the composed system by 3.6 kcal/mole. An almost symmetric equilibrium structure with the hydrogen bonding H-atom midway between the two oxygen centres has been obtained in the CI treatment, whereas SCF calculations yield an asymmetric geometrical configuration with a small energy barrier of 1.4 kcal/mole for the proton transfer process.

A theoretical study of the effects of molecular structure and solvation on tautomeric equilibria in the aromatic and heterocyclic hydroxy and mercaptoa

The SCF-MO-PPP method with Dewar's σ, π- parametrization for ground states of molecules was used to calculate the heats of atomizations, π-electronic distributions and bond lengths of 120 tautomers of aromatic and heterocyclic aldimines. The results obtained are in good agreement with the known experimental data concerning the influence of various structural features on the tautomeric equilibrium states. Some further predictions of preferred forms have been made for those hydroxy and mercaptoaldimines which were not studied earlier. Solvation energies of benzenoid and quinonoid tautomers have been calculated as a function of solvent dielectric permittivity and the influence of the solvent on the tautomeric equilibrium states has been explained. When molecules of quinonoid tautomers are placed in a medium of high dielectric permittivity they have been shown to decline from their equilibrium geometrical structures acquiring the bipolar ion geometrical configurations and thereby causing additional polarizations of the molecules and hence increasing solvation energies. The latter account for the compensations of losses in intrinsic energies. π-Electronic distributions in molecules of free and solvated quinonoid tautomers are discussed.