Search results for "Hybrid functional"
showing 10 items of 45 documents
Vibrational Analysis of Paraelectric–Ferroelectric Transition of LiNbO3: An Ab-Initio Quantum Mechanical Treatment
2021
FSG acknowledges the CINECA award under the ISCRA initiative (HP10BJO47B) for the availability of high-performance computing resources and support.
Multicomponent density-functional theory for electrons and nuclei
2006
We present a general multi-component density functional theory in which electrons and nuclei are treated completely quantum mechanically, without the use of a Born-Oppenheimer approximation. The two fundamental quantities in terms of which our theory is formulated are the nuclear N-body density and the electron density expressed in coordinates referring to the nuclear framework. For these two densities coupled Kohn-Sham equations are derived and the electron-nuclear correlation functional is analyzed in detail. The formalism is tested on the hydrogen molecule $H_2$ and its positive ion $H_2^+$ using several approximations for the electron-nuclear correlation functional.
Orbital-free energy functional for electrons in two dimensions
2009
We derive a non-empirical, orbital-free density functional for the total energy of interacting electrons in two dimensions. The functional consists of a local formula for the interaction energy, where we follow the lines introduced by Parr for three-dimensional systems [R. G. Parr, J. Phys. Chem. 92, 3060 (1988)], and the Thomas-Fermi approximation for the kinetic energy. The freedom from orbitals and from the Hartree integral makes the proposed approximation numerically highly efficient. The total energies obtained for confined two-dimensional systems are in a good agreement with the standard local-density approximation within density-functional theory, and considerably more accurate than …
First-principles investigation of the bulk and low-index surfaces ofMoSe2
2014
In the framework of density functional theory, the geometry, electronic structure, and magnetic properties of the bulk and low index surfaces of $\mathrm{Mo}{\mathrm{Se}}_{2}$ have been studied. We have carried out calculations with various exchange-correlation functionals to select one which is able to describe the van der Waals (vdW) interactions and gives the best geometry compared with experiments. The inclusion of the vdW forces, however, does not guarantee a reliable description for the geometry of this compound: some vdW functionals strongly overestimate the interlayer distance, similar to GGA functionals. Our investigation shows that the recently introduced optB86b-vdW functional yi…
Density gradients for the exchange energy of electrons in two dimensions
2009
We derive a generalized gradient approximation to the exchange energy to be used in density functional theory calculations of two-dimensional systems. This class of approximations has a long and successful history, but it has not yet been fully investigated for electrons in two dimensions. We follow the approach originally proposed by Becke for three-dimensional systems [Int. J. Quantum Chem. 23, 1915 (1983), J. Chem. Phys. 85, 7184 (1986)]. The resulting functional depends on two parameters that are adjusted to a test set of parabolically confined quantum dots. Our exchange functional is then tested on a variety of systems with promising results, reducing the error in the exchange energy b…
Hybrid-Functional Calculations on the Incorporation of Na and K Impurities into the CuInSe 2 and CuIn 5 Se 8 Solar-Cell Materials
2015
International audience; We have studied the energetics, atomic, and electronic structure of Na and K point defects, as well as the (Na-Na), (K-K), and (Na-K) dumbbells in CuInSe2 and CuIn5Se8 solar cell materials by hybrid functional calculations. We found that although Na and K behaves somewhat similar; there is a qualitative difference between the inclusion of Na and K impurities. Namely, Na will be mostly incorporated into CuInSe2 and CuIn5Se8 either as an interstitial defect coordinated by cations, or two Na impurities will form (Na-Na) dumbbells in the Cu sublattice. In contrast to Na, K impurities are less likely to form interstitial defects. Instead, it is more preferable to accommod…
Amino acid chemistry in solution: structural properties and vibrational dynamics of serine using density functional theory and a continuum solvent mo…
2004
A structural and vibrational study of amino acid serine in aqueous solution has been carried out using Fourier transform spectroscopies and quantum mechanical calculations. FT-IR and FT-Raman spectra of serine in H2O and D2O solutions were recorded and a general assignment of the observed bands was proposed on the basis of a zwitterionic structure for serine. Main criteria were the observed wavenumber shifts upon deuteration and previous assignments for other amino acids. A quadratic force field was computed using ab initio methodology at the 6-31+G** level and the hybrid functional B3LYP. The solvent effect was simulated by placing the serine molecule into an ellipsoidal cavity surrounded …
Electronic spectra of tetrathiafulvalene and its radical cation: analysis of the performance of the time-dependent DFT approach
2002
Abstract The electronic spectra of tetrathiafulvalene and its radical cation have been studied within the framework of the time-dependent density functional theory by using a conventional hybrid functional. The behaviour of the method has been analyzed through the computed vertical excitation energies for the low-lying electronic excited states. Although the procedure provides a correct description of many of the features of the spectra, deviations in the range 0.4–0.7 eV have been obtained for several transitions, from which one can conclude misleading assignments.
Nitrogen interstitial defects in silicon. A quantum mechanical investigation of the structural, electronic and vibrational properties
2019
The vibrational features of eight interstitial nitrogen related defects in silicon have been investigated at the first principles quantum mechanical level by using a periodic supercell approach, a hybrid functionals, an all electron Gaussian type basis set and the Crystal code. The list includes defects that will be indicated as Ni (one N atom forming a bridge between two Si atoms), Ni-Ns (one interstitial and one substitutional N atom linked to the same Si atom), Ni-Ni (two Ni defects linked to the same couple of silicon atoms) and Ni-Sii-Ni (two Ni defects linked to the same interstitial silicon atom). Four 〈0 0 1〉 split interstitial (dumbbell) defects have also been considered, in which …
Spin forbidden chemical reactions of transition metal compounds. New ideas and new computational challenges.
2003
International audience; Many reactions of transition metal compounds involve a change in spin. These reactions may proceed faster, slower—or at the same rate as—otherwise equivalent processes in which spin is conserved. For example, ligand substitution in [CpMo(Cl)2(PR3)2] is faster than expected, whereas addition of dinitrogen to [Cp*Mo(Cl)(PMe3)2] is slow. Spin-forbidden oxidative addition of ethylene to [Cp*Ir(PMe3)] occurs competitively with ligand association. To explain these observations, we discuss the shape of the different potential energy surfaces (PESs) involved, and the energy of the minimum energy crossing points (MECPs) between them. This computational approach is of great he…