Pressure dependence of the low-frequency dielectric constant of KNbO_3

The effect of pressure on the low-frequency dielectric constant, $\epsilon_0$, of single crystals of KNbO_3 is investigated by means of capacitance measurements. The dielectric constant increases with pressure up to 22.5 kbar, where it exhibits a large value ($\epsilon_0$ = 5000), and then decreases. This change in its behaviour is related to a phase transition induced by pressure. On decompression, the samples do not revert back to the ambient pressure phase.Comment: 4 pages latex, 1 postscript file include

Implementation of analytical Hartree-Fock gradients for periodic systems

We describe the implementation of analytical Hartree-Fock gradients for periodic systems in the code CRYSTAL, emphasizing the technical aspects of this task. The code is now capable of calculating analytical derivatives with respect to nuclear coordinates for systems periodic in 0, 1, 2 and 3 dimensions (i.e. molecules, polymers, slabs and solids). Both closed-shell restricted and unrestricted Hartree-Fock gradients have been implemented. A comparison with numerical derivatives shows that the forces are highly accurate.Comment: accepted by Comp. Phys. Com

The Raman spectrum of grossular garnet: a quantum mechanical simulation of wavenumbers and intensities

Raman spectroscopy is a standard and powerful investigation technique for minerals, and garnet is one of the most observed and visible minerals, undoubtfully important both as a witness of our planet’s evolution and as a main component in many high-tech applications. This paper presents the Raman spectrum of grossular, the calcium–aluminium end-member of garnets (Ca3Al2Si3O12), as computed by using an ab initio quantum-mechanical approach, an all-electron Gaussian-type basis set and the hybrid B3LYP functional. The wavenumbers of the 25 Raman active modes are in excellent agreement with the available experimental measurements, with the mean absolute difference being between 5 and 8 cm1. The apparent disagreement between a few experimental vs calculated data can be easily justified through the analysis of the corresponding calculated peak intensities, which is very low in all of these cases. The intensities of the Raman active modes of grossular were calculated here for the first time, thanks to a recent implementation by some of the present authors that allow for accurate predictions of the Raman spectra of minerals. To the authors’ knowledge, there are no tabulated data sets for Raman intensities of grossular, although qualitative information can be extracted from the published spectra. This study can then be considered as an accurate reference data set for grossular, other than a clear evidence that quantum-mechanical simulation is an actual tool to predict spectroscopic properties of minerals

Electronic structure and transport properties of atomic NiO spinvalves

Ab-initio quantum transport calculations show that short NiO chains suspended in Ni nanocontacts present a very strong spin-polarization of the conductance. The generalized gradient approximation we use here predicts a similiar polarization of the conductance as the one previously computed with non-local exchange, confirming the robustness of the result. Their use as nanoscopic spinvalves is proposed.Comment: 2 pages, 1 figure; accepted in JMMM (Proceedings of ICM'06, Kyoto

Communication: Hole localization in Al-doped quartz SiO2 within ab initio hybrid-functional DFT

We investigate the long-standing problem of the hole localization at the Al impurity in quartz SiO$_2$, using a relatively recent DFT hybrid-functional method in which the exchange fraction is obtained \emph{ab initio}, based on an analogy with the static many-body COHSEX approximation to the electron self-energy. As the amount of the admixed exact exchange in hybrid functionals has been shown to be determinant for properly capturing the hole localization, this problem constitutes a prototypical benchmark for the accuracy of the method, allowing one to assess to what extent self-interaction effects are avoided. We obtain good results in terms of description of the charge localization and structural distortion around the Al center, improving with respect to the more popular B3LYP hybrid-functional approach. We also discuss the accuracy of computed hyperfine parameters, by comparison with previous calculations based on other self-interaction-free methods, as well as experimental values. We discuss and rationalize the limitations of our approach in computing defect-related excitation energies in low-dielectric-constant insulators.Comment: Accepted for publication in J. Chem. Phys. (Communications

Exploitation of symmetry in periodic Self-Consistent-Field ab initio calculations: application to large three-dimensional compounds

Symmetry can dramatically reduce the computational cost (running time and memory allocation) of Self-Consistent-Field ab initio calculations for crystalline systems. Crucial for running time is use of symmetry in the evaluation of one- and two-electron integrals, diagonalization of the Fock matrix at selected points in reciprocal space, reconstruction of the density matrix. As regards memory allocation, full square matrices (overlap, Fock and density) in the Atomic Orbital (AO) basis are avoided and a direct transformation from the packed AO to the SACO (Symmetry Adapted Crystalline Orbital) basis is performed, so that the largest matrix to be handled has the size of the largest sub-block in the latter basis. We here illustrate the effectiveness of this scheme, following recent advancements in the CRYSTAL code, concerning memory allocation and direct basis set transformation. Quantitative examples are given for large unit cell systems, such as zeolites (all-silica faujasite and silicalite MFI) and garnets (pyrope). It is shown that the full SCF of 3D systems containing up to 576 atoms and 11136 Atomic Orbitals in the cell can be run with a hybrid functional on a single core PC with 500 MB RAM in about 8 h. © 2014 Science China Press and Springer-Verlag Berlin Heidelberg