All-electron Gaussian Basis Sets Research Articles

Optical properties of alkali halide crystals from all-electron hybrid TD-DFT calculations.

We present a study of the electronic and optical properties of a series of alkali halide crystals AX, with A = Li, Na, K, Rb and X = F, Cl, Br based on a recent implementation of hybrid-exchange time-dependent density functional theory (TD-DFT) (TD-B3LYP) in the all-electron Gaussian basis set code CRYSTAL. We examine, in particular, the impact of basis set size and quality on the prediction of the optical gap and exciton binding energy. The formation of bound excitons by photoexcitation is observed in all the studied systems and this is shown to be correlated to specific features of the Hartree-Fock exchange component of the TD-DFT response kernel. All computed optical gaps and exciton binding energies are however markedly below estimated experimental and, where available, 2-particle Green's function (GW-Bethe-Salpeter equation, GW-BSE) values. We attribute this reduced exciton binding to the incorrect asymptotics of the B3LYP exchange correlation ground state functional and of the TD-B3LYP response kernel, which lead to a large underestimation of the Coulomb interaction between the excited electron and hole wavefunctions. Considering LiF as an example, we correlate the asymptotic behaviour of the TD-B3LYP kernel to the fraction of Fock exchange admixed in the ground state functional cHF and show that there exists one value of cHF (∼0.32) that reproduces at least semi-quantitatively the optical gap of this material.

Physico-Chemical Features of Aluminum Hydroxides As Modeled with the Hybrid B3LYP Functional and Localized Basis Functions

Aluminum hydroxides (a family of seven compounds) are key raw materials in the aluminum industry, and indeed an extremely large variety of experiments and simulations was performed in the last 50 years on this class of materials. However, many of their properties still remain a matter of debate, as a comprehensive and comparative analysis of the full family is still lacking. Some of the open questions related to their structure (space group definition, H order–disorder), energetics (relative stability with respect to each other and to the dehydrated phase), and vibrational spectrum (broad bands in the OH stretching region causing large differences in the interpretation of the experimental IR and Raman spectra by the various authors) are addressed in this work with a quantum mechanical ab initio approach, using the CRYSTAL09 periodic code, a rich all-electron Gaussian type basis set, and the hybrid B3LYP functional. This review presents the first complete, systematic, and homogeneous (same method, computational parameters, basis set, and Hamiltonian) investigation of the crystal structure, energetics, and vibrational spectra of this family of compounds, showing that quantum mechanical simulation is likely to provide complementary data for their complete characterization.

Structural and Electronic Properties of Selected Rutile and Anatase TiO2 Surfaces: An ab Initio Investigation.

Five low-index stoichiometric TiO2 rutile and anatase surfaces, i.e., rutile (110), (100), and (001) as well as anatase (101) and (100), have been investigated using different Hamiltonians with all-electron Gaussian basis sets, within a periodic approach. Full-relaxations of the aforementioned surfaces have been essentially carried out at the Hartree-Fock (HF) level, but selected surfaces were treated also using pure and hybrid Density Functional Theory (DFT) models. Mulliken charges, band structures, and total and projected-densities of states have been computed both at the HF and the hybrid DFT (B3LYP and PBE0) levels. As regards DFT, the local density (LDA) and generalized gradient approximations (GGA) have been used. No matter which Hamiltonian is considered, as long as sufficiently thick slabs are taken into account, computed atomic relaxations show an overall excellent agreement with the most recent experimental reports. This is especially true when using hybrid functionals which enable the clarification of some conflicting results. Moreover, both at the LDA and HF levels, we were able to classify the surface relative energies in the following sequence: anatase (101) < rutile (110) < anatase (100) < rutile (100) ≪ rutile (001). Instead, when using PBE, B3LYP, or PBE0, the two most stable surfaces are reversed.

A study of H+H2 and several H-bonded molecules by phaseless auxiliary-field quantum Monte Carlo with plane wave and Gaussian basis sets

The authors present phaseless auxiliary-field (AF) quantum Monte Carlo (QMC) calculations of the ground states of some hydrogen-bonded systems. These systems were selected to test and benchmark different aspects of the new phaseless AF QMC method. They include the transition state of H+H(2) near the equilibrium geometry and in the van der Walls limit, as well as the H(2)O, OH, and H(2)O(2) molecules. Most of these systems present significant challenges for traditional independent-particle electronic structure approaches, and many also have exact results available. The phaseless AF QMC method is used either with a plane wave basis with pseudopotentials or with all-electron Gaussian basis sets. For some systems, calculations are done with both to compare and characterize the performance of AF QMC under different basis sets and different Hubbard-Stratonovich decompositions. Excellent results are obtained using as input single Slater determinant wave functions taken from independent-particle calculations. Comparisons of the Gaussian based AF QMC results with exact full configuration interaction show that the errors from controlling the phase problem with the phaseless approximation are small. At the large basis-size limit, the AF QMC results using both types of basis sets are in good agreement with each other and with experimental values.

Toward a DFT-based molecular dynamics description of Co(ii) binding in sulfur-rich peptides

In this paper, we investigated the reliability of a Car-Parrinello molecular dynamics (CPMD) approach to characterize the binding of Co(II) metal cation to peptide molecules containing cysteine. To this end, we compared pseudo-potentials and DFT plane wave expansion, which are used as key ingredients in the CPMD method, with standard all-electron Gaussian basis set DFT calculations. The simulations presented here are the first attempts to characterize interactions and dynamics of Co(II) metal with the building blocks of phytochelatin peptide molecules. Benchmark calculations are performed on [Co(Cys-H)]+ and [Co(Glutathione-H)]+ complexes, since they are the main fragments of the Co(II)-Cys and Co(II)-glutathione systems found in gas phase electrospray ionisation mass spectrometry (ESI-MS) experiments done in our laboratory. We also present benchmark calculations on the [Co(H2O)6)]2+ cluster with direct comparisons to highly correlated ab initio calculations and experiments. In particular, we investigated the dissociation path of one water molecule from the first hydration shell of Co(II) with CPMD. Overall, our molecular dynamics simulations shed some light on the nature of the Co(II) interaction and reactivity in Co(II)-phytochelatin building block systems related to the biological and environmental activity of the metal, either in the gas or liquid phase.

Contracted all-electron Gaussian basis sets for atoms Rb to Xe

Two series of contracted all-electron basis sets are presented for Rb to Xe which were obtained by full optimization of atomic SCF energies. The smaller, economic basis is of split valence type. The larger is of triple zeta type for the 4d atomic orbitals and better than double zeta quality for remaining valence orbitals.

Muonium as a hydrogen analogue in silicon and germanium: Quantum effects and hyperfine parameters

We report a first-principles theoretical study of hyperfine interactions, zero-point effects, and defect energetics of muonium and hydrogen impurities in silicon and germanium. The spin-polarized density-functional method is used, with the crystalline orbitals expanded in all-electron Gaussian basis sets. The behavior of hydrogen and muonium impurities at both the tetrahedral and bond-centered sites is investigated within a supercell approximation. To describe the zero-point motion of the impurities, a double adiabatic approximation is employed in which the electron, muon/proton, and host lattice degrees of freedom are decoupled. Within this approximation the relaxation of the atoms of the host lattice may differ for the muon and proton, although in practice the difference is found to be slight. With the inclusion of zero-point motion the tetrahedral site is energetically preferred over the bond-centered site in both silicon and germanium. The hyperfine and superhyperfine parameters, calculated as averages over the motion of the muon, agree reasonably well with the available data from muon spin resonance experiments.

Structural, electronic and magnetic properties of KMF3(M=Mn, Fe, Co, Ni)

The structural, electronic and magnetic properties of the perovskite systems KMF3 (M=Mn, Fe, Co, Ni) have been investigated with CRYSTAL95, a periodic abinitio Hartree–Fock program. An all-electron Gaussian basis set has been used. The equation of state has been determined first for the cubic structure; then deviations from cubic symmetry have been explored, with the result that the Mn, Fe and Co systems are found to be slightly more stable in a tetragonal geometry. The systems are almost fully ionic, with net charges for K and M of +1, ca. -0.9 and ca. +1.8 ∣e∣, respectively. The antiferromagnetic (AFM) is always more stable than the ferromagnetic (FM) phase; the energy difference ΔE=E(FM)-E(AFM) is shown: (a) to be additive with respect to the number of M–M first neighbours; (b) to increase with decreasing lattice parameter according to an inverse power law; and (c) to become zero when the M–F–M angle approaches 90°. The super-exchange coupling constants, evaluated from ΔE by using an Ising model hamiltonian, are in qualitative agreement with the experimental data (from 30% to 45% of the latter). Mulliken population data, charge and spin density maps and density of states are used to illustrate the electronic structure.

Theoretical studies of adsorbate binding and ionization of methoxy on the Cu(111) surface

Model studies on small Cu n OCH 3, n = 1, 2, 3 clusters are used to determine the detailed chemisorptive interaction of the methoxy (OCH 3) radical with the Cu(111) surface as well as surface-induced shifts of the adsorbate valence ionization potentials. The calculations are based on the ab initio Hartree-Fock-LCAO method using all-electron Gaussian basis sets. The model results from energy optimizations for various lateral adsorption sites are consistent with experimental findings which suggest that OCH 3 stabilizes at a 3-fold central site on the Cu(111) surface. The OCH 3-metal interaction is characterized by considerable Cu to OCH 3 charge transfer filling the OCH 3 2e lone pair orbitals while OCH 3 to Cu charge transfer involving the OCH 3 5a 1 orbital is small. As a result, the adsorbate becomes negatively charged in the presence of the surface. The calculated adsorbate binding energies ( E B = 1.7 eV for Cu 3OCH 3 with OCH 3 at the central and bridge site, E B = 1.3 eV at the on top site) are found to be rather large containing ionic and covalent contributions of about the same magnitude. Calculations on valence hole states of Cu 3OCH 3 corresponding to ionization of OCH 3-type cluster orbitals are performed. The results show differential shifts of the OCH 3 5a 1 and 2e derived ionization potentials (IP) compared to the free radical values which are explained by the electrostatic adsorbate-substrate interaction and the charge distributions of the orbitals. The reduction of the 5a 1-1e IP separation due to the chemisorptive interaction is consistent with experimental data from photoemission on the OCH 3/Cu(111) adsorbate system.

Interaction of the methoxy radical with copper surfaces: comparative cluster model studies

Model calculations on Cu3OCH3 clusters for various lateral adsorption sites are performed to study the details of the chemisorptive interaction of methoxy (OCH3) with Cu(111) surfaces. The calculations are based on the ab initio Hartree-Fock method using all-electron Gaussian basis sets. The results suggest that OCH3 stabilizes at three-fold central sites of Cu(111) which is consistent with experimental findings. The OCH3-Cu interaction is described by small OCH3 to Cu charge transfer involving the OCH35a1 orbital and a much larger Cu to OCH3 back-transfer filling the OCH32e lone pair orbitals. As a result, the adsorbate species becomes negatively charged in the cluster and the rather large OCH3-Cu binding energy (1.7 eV) contains ionic and covalent contributions of about the same magnitude.