Comparison of Experimental and Theoretical Structure Amplitudes and Valence Charge Densities of GaAs

1998 ◽  
Vol 54 (3) ◽  
pp. 231-239 ◽  
Author(s):  
J. Stahn ◽  
M. Möhle ◽  
U. Pietsch
Keyword(s):  
Ab Initio ◽  
Density Functional ◽  
Theoretical Data ◽  
Charge Densities ◽  
Hartree Fock ◽  
Structure Factors ◽  
Nearest Neighbours ◽  
Kinematic Limit ◽  
Order Of Magnitude ◽  
The One

The current best sets of X-ray structure amplitudes for GaAs, gallium arsenide, are completed by highly precise data recorded at 0.50 < sin θ/λ < 1.35 Å−1. For the strong reflections the required accuracy of ΔF/F ≤ 1% was realized by the use of Pendellösung measurements at λ = 0.30 Å, recording the integral intensities as a function of the effective thickness from ∼500 µm thick GaAs wafers. Additionally, several weak reflections were determined from their integral intensities within the kinematic limit at wavelengths λ = 0.3, 0.56 and 0.71 Å. From these data individual Debye–Waller factors for gallium and arsenic were determined using the model of independent spherical atoms [B Ga = 0.666 (4) and B As = 0.566 (4) Å2]. The extended set of experimental structure factors now available is compared with those obtained by ab initio solid-state Hartree–Fock (HF) and density functional (DF) calculations. Therefore, the theoretical data were adapted to room temperature using the experimentally evaluated Debye–Waller factors and the model mentioned above. The valence and difference charge densities obtained from experimental and theoretical data show the expected charge accumulation between nearest neighbours slightly shifted towards the arsenic site. The disagreement remaining between the experimental and theoretical data, on the one hand, and between those of both ab initio methods, on the other hand, are of the same order of magnitude.

Polaronic and Bipolaronic Enhancement of Second Hyperpolarizabilities in Dithienyl Polyenes from Ab Initio Quantum Methods

1999 ◽  
Vol 597 ◽  
Author(s):  
Steven Trohalaki ◽  
Robert J. Zellmer ◽  
Ruth Pachter
Keyword(s):  
Ab Initio ◽  
Density Functional ◽  
Valence Bond ◽  
Closed Shell ◽  
Functional Theory ◽  
Molecular Conformations ◽  
Hartree Fock ◽  
Molecular Charge ◽  
Field Methodology ◽  
Moller Plesset

AbstractSpangler and He [1,2] have shown that dithienyl polyenes form extremely stable bipolaronic dications when oxidatively doped in solution. Previous theoretical studies applied empirical methods to predict bipolaronic enhancement of hyperpolarizabilities for simple polyenes [3,4]. Here, we employ density functional theory to optimize the gas-phase molecular conformations of neutral, cationic, and dicationic forms of a series of dithienyl polyenes, where the number of ethene units, N, is varied from 1–5. Ab initio Hartree-Fock, generalized valence bond, configuration interaction, and Møller-Plesset calculations demonstrate that the dications are farily well described with a closed shell and therefore have little biradicaloid character. Second hyperpolarizabilities, γ, are subsequently calculated using ab initio Hartree-Fock theory and a finite field methodology. As expected, γ increases with the number of ethene units for a given molecular charge. The cations also show the largest increase in γ with N. For a given value of N, the cations display the largest γ values. However, if we treat the dication as a triplet, which might be present in solution, then it displays the largest γ.

scholarly journals Computational Methods for Ab Initio Molecular Dynamics

2018 ◽  
Vol 2018 ◽  
pp. 1-14 ◽  
Author(s):  
Eric Paquet ◽  
Herna L. Viktor
Keyword(s):  
Molecular Dynamics ◽  
Quantum Mechanics ◽  
Ab Initio ◽  
First Principles ◽  
Density Functional ◽  
Path Integrals ◽  
Ab Initio Molecular Dynamics ◽  
Molecular Systems ◽  
Hartree Fock ◽  
Computational Perspective

Ab initio molecular dynamics is an irreplaceable technique for the realistic simulation of complex molecular systems and processes from first principles. This paper proposes a comprehensive and self-contained review of ab initio molecular dynamics from a computational perspective and from first principles. Quantum mechanics is presented from a molecular dynamics perspective. Various approximations and formulations are proposed, including the Ehrenfest, Born–Oppenheimer, and Hartree–Fock molecular dynamics. Subsequently, the Kohn–Sham formulation of molecular dynamics is introduced as well as the afferent concept of density functional. As a result, Car–Parrinello molecular dynamics is discussed, together with its extension to isothermal and isobaric processes. Car–Parrinello molecular dynamics is then reformulated in terms of path integrals. Finally, some implementation issues are analysed, namely, the pseudopotential, the orbital functional basis, and hybrid molecular dynamics.

scholarly journals EXPANSION FORMULAE FOR ONE- AND TWO-CENTER CHARGE DENSITIES OVER COMPLETE ORTHONORMAL SETS OF EXPONENTIAL TYPE ORBITALS AND THEIR USE IN EVALUATION OF MULTICENTER–MULTIELECTRON INTEGRALS

2009 ◽  
Vol 08 (04) ◽  
pp. 597-602 ◽  
Author(s):  
I. I. GUSEINOV
Keyword(s):  
Exponential Type ◽  
Overlap Integrals ◽  
Charge Densities ◽  
Hartree Fock ◽  
Expansion Formulae ◽  
Multicenter Multielectron Integrals ◽  
Explicitly Correlated ◽  
Explicitly Correlated Methods ◽  
Exponential Type Orbitals ◽  
The One

The series expansion formulae are established for the one- and two-center charge densities over complete orthonormal sets of Ψα-exponential type orbitals (Ψα-ETO α = 1,0,-1,-2,…) introduced by the author. Three-center overlap integrals of Ψα appearing in these relations are expressed through the two-center overlap integrals between Ψα-orbitals. The general formulae obtained for the charge densities are utilized for the evaluation of arbitrary multicenter–multielectron integrals occurring when the complete orthonormal sets of Ψα-ETO are used as basis functions in the Hartree–Fock–Roothaan and explicitly correlated methods. The relationships for charge densities and multicenter–multielectron integrals obtained are valid for the arbitrary quantum numbers, screening constants, and location of Ψα-orbitals.

Theoretical studies on the one- and two-photon absorption of tetrabenzoporphyrins and phthalocyanines

2004 ◽  
Vol 82 (1) ◽  
pp. 19-26 ◽  
Author(s):  
Xin Zhou ◽  
Ai-Min Ren ◽  
Ji-Kang Feng ◽  
Xiao-Juan Liu
Keyword(s):  
Dipole Moment ◽  
Cross Sections ◽  
Density Functional ◽  
Transition Dipole Moment ◽  
Photon Absorption ◽  
Transition Dipole ◽  
Two Photon Absorption ◽  
Two Photon ◽  
Order Of Magnitude ◽  
The One

The one-photon absorption (OPA) properties of tetrabenzoporphyrins (TBPs) and phthalocyanines (Pcs) were studied using the semiempirical ZINDO method and time-dependent density functional theory (TDDFT), respectively. The compared results confirmed that the semiempirical ZINDO method was reasonably reliable when calculating the OPA of tetrabenzoporphyrins and phthalocyanines. On the basis of the OPA properties obtained from the ZINDO method, two-photon absorption (TPA) properties of two series of molecules were investigated, using ZINDO and sum-over-states (SOS) methods. The results showed that the TPA cross-sections of all molecules were in the range of 220.6 × 10–50 – 345.9 × 10–50 cm4·s·photon–1, which were in the same order of magnitude as the values reported in the literature. The relatively larger δ(ω) value for Pcs with respect to that for corresponding TBPs originates from larger intramolecular charge transfer, which can be characterized by the difference of dipole moment between S0 and S1 and the transition dipole moment between S1 and S5.Key words: two-photon absorption, ZINDO, sum-over-states, tetrabenzoporphyrin, phthalocyanines.

scholarly journals QM AND AB INITIO INVESTIGATION ON THE HYDROGEN BONDING, NMR CHEMICAL SHIFTS AND SOLVENT EFFECTS ON THE DPPE

2010 ◽  
Vol 7 (3) ◽  
pp. 260-272
Author(s):  
M. Monajjemi ◽  
A. Nouri ◽  
H. Monajemi
Keyword(s):  
Hydrogen Bonding ◽  
Ab Initio ◽  
Atomic Orbital ◽  
Theoretical Data ◽  
Basis Sets ◽  
Stable Complex ◽  
Head Group ◽  
Water Molecules ◽  
Hartree Fock ◽  
Zwitterionic Form

The hydrogen bonding effects that were produced from interaction of membrane lipid dipalmitoylphosphatidyl-ethanolamine (DPPE) with 1-5 water molecules, has been theoretically  investigated through the quantum mechanical calculations at the Hartree-Fock level of theory and the 3-21G, 6-31G and 6-31G* basis sets with the computational package of Gaussian 98. According to the obtained results of the structural optimization of the isolated DPPE in the gas phase, we can see the evidences of interactions in the head group of this macromolecule (from the molecular point of view we have a proton transfer from the ammonium group to the phosphate oxygen of zwitterionic form. As we know that the hydrogen bonding of DPPE with water molecules which have surrounded its head group plays an important role in the permeability of DPPE. So, in order to understand the microscopic physico-chemical nature of this subject we have analyzed bond and torsion angles of DPPE before and after added water molecules.  In this paper we have theoretically studied the complexes DPPE with water molecules which have surrounded its head group. As mentioned before, this theoretically study has been done through Hartree-Fock level of theory by using simple basis sets. Theoretical data shows that the interaction of head group of DPPE with water molecules causes some changes in the geometry of DPPE which were explained by the contribution of zwitterionic form of DPPE macromolecule, and finally hydrated DPPE becomes stable complex. Comparison between theoretical and experimental geometry data of DPPE macromolecule shows that the calculation at the HF/3-21 level of theory produces results which they are in better agreement with the experimental data. Moreover the hydrogen bonding effects on the NMR shielding tensor of selected atoms in the hydrated complexes of DPPE were reported. The ";Gauge Including Atomic Orbitals"; (GIAO) approaches within the SCF-Hartree-Fock approximation have been used in order to investigate the influence of hydrogen bonding of DPPE-water complex on the shielding tensors. Finally, the solvent affects on the stability of DPPE macromolecule, dipole moment and atomic charge of some selected atoms of DPPE molecule was discussed using Onsager model and Merz-Singh-Kolman schema.   Keywords  : Gauge Including Atomic Orbital, DPPE, hydrogen bonding, solvation, quantum mechanics, ab initio

scholarly journals Investigation of the nuclear structure of 84-108Mo isotopes using Skyrme-Hartree-Fock method

2019 ◽  
Vol 13 (26) ◽  
pp. 1-11
Author(s):  
Ali A. Alzubadi
Keyword(s):  
Density Functional ◽  
Good Description ◽  
Mean Field ◽  
Nucleon Nucleon ◽  
Effective Density ◽  
Skin Thickness ◽  
Neutron Skin ◽  
Energy Density Functional ◽  
Charge Densities ◽  
Hartree Fock

Over the last few decades the mean field approach using selfconsistentHaretree-Fock (HF) calculations with Skyrme effectiveinteractions have been found very satisfactory in reproducingnuclear properties for both stable and unstable nuclei. They arebased on effective energy-density functional, often formulated interms of effective density-dependent nucleon–nucleon interactions.In the present research, the SkM, SkM*, SI, SIII, SIV, T3, SLy4,Skxs15, Skxs20 and Skxs25 Skyrme parameterizations have beenused within HF method to investigate some static and dynamicnuclear ground state proprieties of 84-108Mo isotopes. In particular,the binding energy, proton, neutron, mass and charge densities andcorresponding root mean square radius, neutron skin thickness andcharge form factor are calculated by using this method with theSkyrme parameterizations mentioned above. The calculated resultsare compared with the available experimental data. Calculationsshow that the Skyrme–Hartree–Fock (SHF) theory with aboveforce parameters provides a good description on Mo isotopes.

scholarly journals Vibrationally and Spin-Orbit-Resolved Inner-Shell X-ray Absorption Spectroscopy of the NH+ Molecular Ion: Measurements and ab Initio Calculations

Atoms ◽  
2020 ◽  
Vol 8 (4) ◽  
pp. 67
Author(s):  
Stéphane Carniato ◽  
Jean-Marc Bizau ◽  
Denis Cubaynes ◽  
Eugene T. Kennedy ◽  
Ségolène Guilbaud ◽  
...  
Keyword(s):  
Ab Initio ◽  
Density Functional ◽  
Theoretical Calculations ◽  
High Sensitivity ◽  
Open Shell ◽  
Field Configuration ◽  
High Spectral Resolution ◽  
Spin Orbit ◽  
Molecular Ion ◽  
Hartree Fock

This article presents N2+ fragment yields following nitrogen K-shell photo-absorption in the NH+ molecular ion measured at the SOLEIL synchrotron radiation facility in the photon energy region 390–450 eV. The combination of the high sensitivity of the merged-beam, multi-analysis ion apparatus (MAIA) with the high spectral resolution of the PLEIADES beamline helped to resolve experimentally vibrational structures of highly excited [N1s−1H]*+ electronic states with closed or open-shell configurations. The assignment of the observed spectral features was achieved with the help of density functional theory (DFT) and post-Hartree Fock Multiconfiguration Self-Consistent-Field/Configuration Interaction (MCSCF/CI) ab-initio theoretical calculations of the N1s core-to-valence and core-to-Rydberg excitations, including vibrational dynamics. New resonances were identified compared to previous work, owing to detailed molecular modeling of the vibrational, spin-orbit coupling and metastable state effects on the spectra. The latter are evidenced by spectral contributions from the 4Σ− electronic state which lies 0.07 eV above the NH+2Π ground state.

scholarly journals Density functional tight binding: values of semi-empirical methods in an ab initio era

2014 ◽  
Vol 16 (28) ◽  
pp. 14368-14377 ◽  
Author(s):  
Qiang Cui ◽  
Marcus Elstner
Keyword(s):  
Density Functional Theory ◽  
Ab Initio ◽  
Density Functional ◽  
Tight Binding ◽  
Empirical Methods ◽  
Functional Theory ◽  
Hartree Fock ◽  
Semi Empirical

Semi-empirical (SE) methods are derived from Hartree–Fock (HF) or Density Functional Theory (DFT) by neglect and approximation of electronic integrals.

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