Search results for "Ab initio quantum chemistry methods"
showing 10 items of 468 documents
High-precision ab initio calculations of the spectrum of Lr$^+$
2019
The planned measurement of optical resonances in singly-ionised lawrencium (Z = 103) requires accurate theoretical predictions to narrow the search window. We present high-precision, ab initio calculations of the electronic spectra of Lr$^+$ and its lighter homologue lutetium (Z = 71). We have employed the state-of-the-art relativistic Fock space coupled cluster approach and the AMBiT CI+MBPT code to calculate atomic energy levels, g-factors, and transition amplitudes and branching-ratios. Our calculations are in close agreement with experimentally measured energy levels and transition strengths for the homologue Lu$^+$ , and are well-converged for Lr$^+$ , where we expect a similar level o…
The fate of the resonating valence bond in graphene
2011
We apply a variational wave function capable of describing qualitatively and quantitatively the so called "resonating valence bond" in realistic materials, by improving standard ab initio calculations by means of quantum Monte Carlo methods. In this framework we clearly identify the Kekul\'e and Dewar contributions to the chemical bond of the benzene molecule, and we establish the corresponding resonating valence bond energy of these well known structures ($\simeq 0.01$eV/atom). We apply this method to unveil the nature of the chemical bond in undoped graphene and show that this picture remains only within a small "resonance length" of few atomic units.
Wavelength dependence of multiphoton ionization of xenon
2004
We have studied the multiphoton ionization of xenon atoms by $160\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulses at intensities of $5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ and $1.3\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ and present photoelectron kinetic energy and angular distribution spectra measured with a photoelectron imaging spectrometer. A noncollinear optical parametric amplifier allows us to tune the wavelength of the laser pulse over a range between 500 and $700\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Resonant and nonresonant processes as well as channel switching effects have been observed in this intensity and wavel…
Diagrammatic expansion for positive spectral functions beyond GW : Application to vertex corrections in the electron gas
2014
We present a diagrammatic approach to construct self-energy approximations within many-body perturbation theory with positive spectral properties. The method cures the problem of negative spectral functions which arises from a straightforward inclusion of vertex diagrams beyond the GW approximation. Our approach consists of a two-steps procedure: we first express the approximate many-body self-energy as a product of half-diagrams and then identify the minimal number of half-diagrams to add in order to form a perfect square. The resulting self-energy is an unconventional sum of self-energy diagrams in which the internal lines of half a diagram are time-ordered Green's functions whereas those…
The Dalton quantum chemistry program system
2013
Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree-Fock, Kohn-Sham, multiconfigurational self-consistent-field, MOller-Plesset, confi ...
π conjugation across the tetrathiafulvalene core: Synthesis of extended tetrathiafulvalene derivatives and theoretical analysis of their unusual elec…
2000
A series of extended tetrathiafulvalene (TTF) derivatives bearing one or two 1,4-dithiafulven-6-yl substitutents has been prepared. The new compounds present remarkable electrochemical singularities compared with other TTF derivatives, which are strongly affected by the nature of the substitution on the lateral heterocycle(s). This unusual electrochemical behaviour follows a square-scheme sequence and is attributed to structural changes upon oxidation of the pi-donating molecules. Digital simulations of the electrochemical data have been used to reach the values of the kinetic and thermodynamic constants involved in the square scheme. Theoretical calculations establish an important contribu…
Fourier-transform spectroscopy and description of low-lying energy levels in the B(1)(1)Π state of RbCs.
2013
The Fourier transform spectrometer with resolution of 0.03 cm(-1) was applied to disperse the diode laser induced B(1)(1)Π → X(1)Σ(+) fluorescence spectra of the RbCs molecule in a heat pipe. The presence of buffer gas (Ar) produced in the spectra the dense pattern of collision-induced rotation relaxation lines, thus enlarging the B(1)(1)Π data set, allowing to determine the Λ-splitting constants and to reveal numerous local perturbations. In total, 2664 term values for (85)Rb(133)Cs and (87)Rb(133)Cs in the energy range from 13,770 to 15,200 cm(-1) were obtained with accuracy about 0.01 cm(-1). A pointwise potential energy curve (PEC) based on inverted perturbation approach was constructed…
Potential construction of the B (1) 1 Π state in KCs based on Fourier-Transform spectroscopy data
2015
Abstract The paper presents an empirical pointwise potential energy curve (PEC) of the extensively perturbed B ( 1 ) 1 Π state of the KCs molecule constructed by applying an Inverted Perturbation Approach routine. The experimental term values in the energy range E ( v ′ , J ′ ) ∈ [ 14071 ; 15502 ] cm − 1 involved in the fit were based on Fourier-Transform spectroscopy data obtained with 0.01 cm−1 accuracy from laser-induced B ( 1 ) 1 Π → X 1 Σ + fluorescence spectra in the present work (654 term values) combined with 520 term values from Birzniece et al. (2012) . The data set included vibrational v ′ ∈ [ 0 , 35 ] and rotational J ′ ∈ [ 7 , 233 ] quantum numbers covering about 85% of the pot…
B1(1)Π state of KCs: high-resolution spectroscopy and description of low-lying energy levels.
2012
The diode laser induced B(1)(1)Π → X(1)Σ(+) fluorescence spectra of the KCs molecule were recorded by Fourier-transform spectrometer with resolution of 0.03 cm(-1). Buffer gas Ar was used to facilitate appearance of rotation relaxation lines in the spectra, thus enlarging the B(1)(1)Π dataset, allowing one to determine the Λ-splitting constants and to reveal numerous local perturbations. A dataset was systematically obtained for B(1)(1)Π vibrational levels ν(') ∈ [0; 5] nonuniformly covering rotational levels with J(') ∈ [7; 233]. For ν(') ∈ [0; 3], the less-perturbed data of f-components were included in the fit. A pointwise potential energy curve (PEC) based on the inverted perturbation a…
Modelling of defects and surfaces in perovskite ferroelectrics
2003
The results of electronic structure calculations for different terminations of SrTiO 3 (100) and (110) perovskite thin films are discussed. These calculations are based on the ab initio Hartree-Fock (HF) method and Density Functional Theory (DFT). Results are compared with previous ab initio plane-wave LDA and classical Shell Model (SM) calculations. Calculated considerable increase of the Ti-O chemical bond covalency nearby the surface is confirmed by experimental data. Our quantum chemical calculations performed by means of the intermediate neglect of differential overlap (INDO) method confirm the existence of self-trapped electrons in KNbO 3 , KTaO 3 and BaTiO 3 crystals. The relevant la…