Search

The IR and Raman spectra of the Fe(3)Al(2)Si(3)O(12) almandine garnet were simulated using the periodic CRYSTAL code with an all-electron Gaussian-type basis set and the hybrid B3LYP functional. The wavenumbers of the 25 Raman-active modes (3 of A(1g), 8 of E(g), and 14 of F(2g) symmetry) and 34 F(1u) IR-active modes (17 TO and 17 LO) were computed, as were the IR intensities. Calculated wavenumbers are in excellent agreement with the various sets of experimental results, with the mean absolute difference |Delta| being between 4 and 8 cm(-1). Graphical animation, available on the CRYSTAL Web site, provides a very comprehendible description of the movement of atoms and groups in each vibrational mode. The simulated reflectivity spectrum, obtained using a classical dispersion relation, is in excellent agreement with the measured one.

The ground state properties of fluorite (CaF2) have been studied using CRYSTAL, an ab initio periodic Hartree-Fock program. Twenty-two and thirteen atomic orbitals (represented as contracted Gaussian-type functions) are used for the calcium and fluorine atoms, respectively. The binding energy (BE), the equilibrium lattice parameter (a), the elastic constants (Cij) and the central zone phonon frequencies nu (IR) and nu (Raman) have been evaluated, and a good agreement obtained with experiment (for instance the error is +2.0, +1.7, -0.5% for BE, a and C11, respectively). The calculated C44 elastic constant reduces from 48 to 44 GPa (experimental value: 37 GPa) when the fluorine atoms are allowed to displace under strain, indicating the importance of inner deformation for shear elasticity. Electron density maps, density of states and band structure plots are reported which confirm the fully ionic nature of fluorite.

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the ground-state structural, electronic, and magnetic properties of the rutile-type compounds MF2 (M=Mn, Fe, Co, and Ni). All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the [001], [111], and [100] (or [010]) directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed.

The electronic structure of A2CuO2X2 (A5Ca, Sr and X5F ,Cl) compounds is investigated by means of the periodic unrestricted Hartree-Fock ~UHF! method using the linear combination of atomic orbitals approach ~LCAO!. The relative stability of different alternative structures is discussed. All the systems are described as insulators with strong ionic character and well localized spin moments on the Cu atoms. The O 2p nature of the highest occupied bands is clear, supporting the charge transfer nature of this kind of narrow band systems; optical gaps are however, overestimated. The magnetic ordering of these materials is two dimensional in nature with almost independent antiferromagnetic CuO2 planes with Jc /J1;1023. The in–plane nearest (J1) and next-nearest (J2) magnetic coupling constants are antiferromagnetic and much larger than the interplane interactions (Jc), with J2 /J1;0.02. The relative values of the magnetic constants are in qualitative agreement with available experimental results. None of the analyzed properties provide differences between the various compounds so relevant to limit the possible development of a superconducting phase. On the contrary, the close similarity between them suggest that kinetic limitations in the doping process can be responsible of the fact that no superconducting transition has been observed in Sr2CuO2Cl2 and in the T8 phase of Sr2CuO2F2 and Ca2CuO2F2.

FSG acknowledges the CINECA award under the ISCRA initiative (HP10BJO47B) for the availability of high-performance computing resources and support., The phase transitions between paraelectric (PE) and ferroelectric (FE) isomorph phases of LiNbO3 have been investigated quantum mechanically by using a Gaussian-type basis set, the B3LYP hybrid functional and the CRYSTAL17 code. The structural, electronic and vibrational properties of the two phases are analyzed. The vibrational frequencies evaluated at the Γ point indicate that the paraelectric phase is unstable, with a complex saddle point with four negative eigenvalues. The energy scan of the A2u mode at −215 cm−1 (i215) shows a dumbbell potential with two symmetric minima. The isotopic substitution, performed on the Li and Nb atoms, allows interpretation of the nontrivial mechanism of the phase transition. The ferroelectric phase is more stable than the paraelectric one by 0.32 eV. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.--//-- Published under CC BY 4.0 licence., CINECA HP10BJO47B; Institute of Solid State Physics, University of Latvia as the Center of Excellence is supported through the Framework Program for European universities Union Horizon 2020, H2020-WIDESPREAD-01–2016–2017-TeamingPhase2 under Grant Agreement No. 739508, CAMART2 project.

The vibrational spectrum of Mg2SiO4 olivine was calculated at the Γ point by using the periodic ab initio CRYSTAL program. An all electron localized Gaussian-type basis set and the B3LYP Hamiltonian were employed. The full set of frequencies (35 IR active, 36 Raman active, 10 "silent" modes) was computed and compared to experimental data from different sources (four for IR and four for Raman). A generally good agreement is observed with experiment (the mean absolute difference ranging from 7 to 10 cm-1 for the various sets), when some of the experimental frequencies, whose attribution is uncertain or appears to be affected by large errors, are not taken into account. A small number of observed peaks are not consistent with calculated frequencies, and a few theoretical peaks do not correspond to measured values. The implications are discussed in detail. The full set of modes are characterized using different tools, namely isotopic substitution, direct inspection of the eigenvectors and graphical representation, so as to obtain a consistent mode assignment. © Springer-Verlag 2006.

International audience; The vibrational spectrum of Ca3Fe2Si3O12 andradite is calculated at the Γ point by using the periodic ab initio CRYSTAL program that adopts an all-electron Gaussian-type basis set and the B3LYP Hamiltonian. The full set of frequencies (17 IR active, 25 Raman active, and 55 inactive modes) is calculated. The effect of the basis set on the calculated frequencies is discussed. The modes are characterized by direct inspection of the eigenvectors and isotopic substitution. The present calculations permit us to clarify some of the assignment problems raised by experiments. The mean absolute differences of the various modes with respect to the available experimental IR and Raman data are as small as 9 and 5 cm-1, respectively.

The five piezoelectric stress coefficients eik of orthorhombic Im 2m NaNO2 were calculated by ab initio quantum mechanical methods, employing the Berry phase theory, with an all-electron basis set of localized Gaussian-like functions and either a DFT-GGA or a Hartree-Fock Hamiltonian. DFT results are larger by about 30% than HF ones. The purely ionic and electronic contributions to piezoelectricity were evaluated by computing the internal and external (clamped-ion), respectively, components of each coefficient. In the e34 case the clamped-ion component is dominant, indicating that the electronic contribution prevails over the ionic one of opposite sign. In all other cases the overall piezoelectric effect is ruled by the ionic contribution. The full set of elastic constants was also computed, by means of which the piezoelectric strain coefficients dik could be derived. These are discussed and compared to experimental data from the literature. © 2005 IOP Publishing Ltd.

The harmonic frequency spectrum of bulk Mg(OH)(2), brucite, has been computed with the CRYSTAL periodic code, using four different hamiltonians, namely Hartree-Fock (HF), local density (LDA), gradient corrected PW91 and hybrid B3LYP. The anharmonicity of the 011 stretching frequency has also been evaluated, as well as the transverse/longitudinal optical (TO/LO) splitting. Comparison with the frequencies of the single layer shows that the bulk OH stretching splitting is due to interlayer interaction. Agreement with experiment is excellent for B3LYP (less than 10 cm(-1) differences), less so for PW91 and much less satisfactory for both LDA and HF. (C) 2004 Elsevier B.V. All rights reserved

The present study discusses the main features of the two programs CRYSTAL and EMBED developed for the ab initio study of the electronic properties of perfect periodic structures and of crystals with local defects, respectively. After a brief historical introduction, the structure of CRYSTAL is outlined and some specific aspects are discussed in detail: the use of local basis functions, the way of dealing with the Coulomb and the exchange series, the exploitation of point symmetry, the possibility to adopt either the Hartree–Fock approach or one among a variety of Kohn–Sham Hamiltonians. The present capabilities of the program are illustrated by a survey of selected applications from existing literature. Information is provided concerning work in progress aimed at removing some of the limitations of the code and improving its performance. The characteristics of EMBED are analyzed with emphasis given to the critical aspects of the method: limits of validity of the fundamental approximation on which the embedding technique relies, problems of convergence of the self-consistent procedure; and the delicate issue of the estimate of the defect formation energy. Again, a critical survey of applications clarifies the capabilities of the code. The envisaged improvements to be introduced in a forthcoming release of EMBED are presented. The present and prospective role of the two programs in the field of computational studies of condensed matter problems is outlined. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 1032–1048, 2000