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Ab initio calculations of the atomic and electronic structure of MgF2 (011) and (111) surfaces

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Vassilyeva A., Eglitis R., Kotomin E., Dauletbekova A.
Open Physics
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2011
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vol. 9
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issue 2
515-518
EN
The results of ab initio slab calculations of surface relaxations, rumplings and charge distribution for the different terminations of the MgF2 (011) and (111) polar surfaces are presented and discussed. We have employed the computer code CRYSTAL with the Gaussian basis set and the hybrid B3PW exchange-correlation functional. Despite the ionic nature of the chemical bonding at both surfaces, a considerable decrease of the optical band gap is predicted (1.3 eV or 10%) for the (111) surface as compared to the bulk.
2
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A first-principles studies on TlX (X=P, As)

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Ciftci Y., Colakoglu K., Deligoz E.
Open Physics
|
2008
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vol. 6
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issue 4
802-807
EN
We present an ab initio study of the structural, electronic and thermodynamic properties of TlX(X=P,As). The plane-wave pseudopotential approach to the density-functional theory within the LDA and GGA approximations implemented in VASP (Viena Ab-initio Simulation Package) is used. The calculated lattice parameter, elastic constants, and band structures are compared with other available theoretical results, and good agreement is obtained. In addition, we have calculated the transition pressure (P t) from zinc-blende (ZB) to (rock-salt) NaCl structures, and have examined some thermodynamic properties.
3
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Ab initio studies of two pyrimidine derivatives as possible photo-switch systems

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Csehi A., Woywod C., Halász G., Vibók Á.
Open Physics
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2013
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vol. 11
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issue 9
1141-1148
EN
The six lowest lying electronic singlet states of 8-(pyrimidine-2-yl)quinolin-ol and 2-(4-nitropyrimidine-2-yl)ethenol have been studied theoretically using the complete active space self-consistent-field (CASSCF) and M’ller-Plesset second-order perturbation theory (MP2) methods. Both molecules can be viewed as consisting of a frame and a crane component. As a possible mechanism for the excited-state relaxation process an intramolecular hydrogen transfer promoted by twisting around the covalent bond connecting the molecular frame and crane moieties has been considered. Based on this idea we have attempted to derive abstracted photochemical pathways for both systems. Geometry optimizations for the construction of hypothetical reaction coordinates have been performed at the MP2 level of theory while the CASSCF approach has been employed for the calculation of vertical excitation energies along the pathways. The results of the calculations along the specific twisting displacements investigated in this study do not support the notion of substantial twisting activity upon excitation of any of the five excited states at the planar terminal structures of the torsion coordinates of both molecules. However, the present analysis should be considered only as a first, preliminary step towards an understanding of the photochemistry of the two candidate compounds. For example, we have not performed any excited state geometry optimizations so far and the estimates of vertical excitation energies do not take dynamical electron correlation into account. Further work on this subject is in progress.
4
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Solid-state linear-dichroic infrared (IR-LD) spectroscopic and theoretical characterization of 2-[(2-ethoxy-3,4-dioxocyclobut-1-en-yl)amino]propanamide

100%
Zareva S.
Open Chemistry
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2006
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vol. 4
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issue 4
695-707
EN
2-[(2-Ethoxy-3,4-dioxocyclobut-1-en-yl)amino]propanamide (N-alaninamidoamide of squaric acid ethyl ester) has been characterized structurally and spectroscopically by ab initio calculations and IR-LD spectroscopy of oriented crystals suspended in a nematic liquid crystal. The results are compared with single crystal X-ray structures illustrating the possibilities of this experimental approach to obtaining structural information as well as assigning IR bands.
5
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First-principles investigation of the vibrational properties of 3-tert-butylcyclohexene

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Gaál-Nagy K.
Open Physics
|
2010
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vol. 8
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issue 5
833-842
EN
I present a first-principles investigation of the vibrational properties of the chiral molecule 3-tert-butylcyclohexene. The vibrational density of states (vDOS) of the two existing conformers has been calculated ab initio within the framework of density-functional theory and density-functional perturbation theory, using both the local-density approximation and the generalized-gradient approximation for the exchange-correlation potential. The vDOS of the two conformers are very similar. The vDOS has been investigated with respect to contributions of the cyclohexene ring and the tert-butyl group and also regarding the localization of vibrational modes. Additionally, the eigendisplacements of characteristic modes of 3-tert-butylcyclohexene have been analyzed.
6
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Electronic structure of Ti-doped Sr4Sc2Fe2As2O6 as a possible parent phase for the new FeAs superconductors

100%
Shein I., Ivanovskii A.
Open Physics
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2010
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vol. 8
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issue 3
432-437
EN
First principle FLAPW-GGA calculations have been performed with the purpose to understand the effect of Ti-doping on the electronic properties for the newly discovered tetragonal iron arsenide-oxide Sr4Sc2Fe2As2O6 (abbreviated as FeAs42226) as the possible parent phase for the new FeAs superconductors. Our results show that the insertion of Ti into Sc sublattice of this five-component iron arsenide-oxide phase leads to the resolute change of electronic structure of FeAs42226. Namely, the insulating oxygen-containing [Sr4Sc2O6] blocks in Ti-doped FeAs42226 became conducting, and this differs essentially from the known picture for all others FeAs superconductors where the conducting [Fe2As2] blocks are alternated with insulating blocks. Moreover in sharp contrast with FeAs-based superconductors with Fe 3d bands near the Fermi level, for Ti-doped FeAs42226 in this region the Ti 3d states are dominated, whereas the Fe 3d states are suppressed.
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