Nicolae M. Avram

In this paper we investigated the impurity center local structure by simulation of the structural proprieties of the rare-earth impurity centers RE³⁺/Bi³⁺ in lanthanum zirconated La₂Zr₂O₇. Rare-earth ion RE³⁺/Bi³⁺ replaces the cation La³⁺ and determines distortions of the La₂Zr₂O₇ crystal lattice near the impurity center. These distortions characterized by radial and angular displacements of anions were investigated by ab initio calculations. We have employed the MO LCAO approach, in the frame of DFT method as it is implemented in the CRYSTAL17 software for periodic systems, using the hybrid functional PBE0. Since the main contribution to the crystal field on the rare-earth impurity ions comes from its nearest neighbors, the distance “rare-earth ion – ligand” and coordinates of the ligands (oxygen ions) near the impurity centers are calculated. The obtained results are discussed and compared with experimental data and the agreement is quite satisfactory.

Herein, we study the electronic structure, energies, and vibronic structure of optical d-d transitions of Cr³⁺ ions doped in beryl (Be₃Si₆Al₂O₁₈:Cr³⁺, emerald). A computational protocol is developed that combines periodic density functional theory (for modeling of the bulk crystalline lattice of emerald) and the multireference configuration interaction complete active space self-consistent field method supplemented with n-electron valence second-order perturbation theory (for the calculation of the energy levels, wave functions, and spin-Hamiltonian and ligand-field parameters of the trigonal Cr³⁺ centers in the [CrO₆]^9– clusters embedded in an extended point charge field). Ligand-field parameters were extracted from mapping the effective ligand-field Hamiltonian onto the full many-particle Hamiltonian from one side and from a direct fit to energies of computed d-d transitions on the other side. These have been analyzed using ab initio ligand-field theory. The quality of the theoretical predictions is critically assessed through a detailed comparison with the available experimental data.

The present paper reports a theoretical investigation based on first-principles density functional theory calculations to predict the external morphology of the tetragonal GdVO₄crystal from its internal structure. The Bravais–Friedel–Donnay–Harker (BFDH) method, attachment energy (AE) method and surface energy (SE) method were used in this study. Slice energies (cohesive, attachment and specific surface) of the three main crystal faces having (110), (101) and (200) orientation and theird_hklthicknesses were computed usingCRYSTAL17code, in the frame of a 2D periodic slab model. The relative growth rate (R_hkl) and the morphological importance (MI_hkl) for each unrelaxed and relaxed (hkl) face of interest were determined. Consequently, the crystal shapes predicted based upon BFDH, AE and SE methods were represented by the Wulff construction. The results of the morphology crystal predictions, based on the above methods, were compared both against each other and against the experimentally observed morphologies. A quite satisfactory agreement between the predicted and observed crystal morphologies is noticed.

Within the framework of an unified ab initio approach the crystal structure, the effect of hydrostatic pressure on the crystal structure, and the phonon spectra of crystals FeBO₃ and GaBO₃ were investigated. Also, an analysis of the displacement vectors has been obtained from the ab initio calculation, which made it possible to estimate the degree of the ion displacements in each vibrational mode. Calculations were carried out within the framework of MO LCAO approach and the density functional theory, by using PBE0 hybrid functional which takes into account both the local and nonlocal (in the Hartree–Fock formalism) exchange. The calculations were performed with CRYSTAL14 software designed to simulate periodic structures. The results of the performed calculations suggest new features of the title crystals under pressure and new vibrational properties.

In this paper we present, in the unified frame, the results of the ab initio investigations of absorption spectra, ligand field parameters and g factors for three valence chromium doped α -Al ₂ O ₃ crystal. Our calculations are based on a new methodology applied to a cluster [CrO ₆ ] ⁹⁻ embedded in an extended point charge field of host matrix ligands. After the differential functional theory optimization of the doped crystal, a vibrational theoretical spectroscopic study based on infrared spectroscopy has been employed in order to confirm the stability of the optimized doped crystal structure. The ab initio energy calculation of the electronic states and corresponding wave functions of Cr ³⁺ are documented from the complete active space self-consistent field. The improved energy states from the N-electron valence second order perturbation theory, second order dynamic correlation dressed complete active space (DCD-CAS2), difference dedicate configuration interaction with three degrees of freedom (MRDDCI3) and spectroscopy-oriented configuration interactions, were analyzed. Based on the ab initio ligand field theory procedure we extracted ligand field parameters and spin–orbit coupling constant which were used to recalculate the energy levels of the studied system. In addition, g factors for the ground state ⁴ A ₂ of Cr ³⁺ ion in corundum are calculated taking into account the full configuration interaction. The results obtained are discussed and the comparisons with measured values from literature show a reasonable agreement, which justifies and recommend this new route of investigation.

The aim of this paper is to present results of both experimental and theoretical vibrational frequencies of normal modes and its vibrational assignments for betulinic acid methanol monosolvate molecule. FTIR spectra were recorded and collected in 4000-400 cm⁻¹ spectral range with a resolution of 2 cm⁻¹. For calculations, the molecule extraction from betulinic acid methanol monosolvate molecular crystal was based on typical fragmentation scheme and chemical connectivity. Knowing geometry of molecular crystal, the obtained geometry of molecule has been optimized and DFT calculation regarding IR frequencies for all 255 normal modes was calculated. All computations has been done using for all electrons a localized basis set of Gaussian type functions TZVP with local and gradient corrected functional BP86. Our calculated frequencies are compared with available experimental data and the agreement is quite satisfactory. Overall, this justifies the simulation scheme used for the title system.