Importance Of Electron Correlation Research Articles

Charge distribution, electric multipole moments, static polarizability and hyperpolarizability of silene

We have obtained electric properties for silene from finite-field, high level ab initio calculations with molecule-specific Gaussian basis sets. Our best values, calculated at the CCSD(T) level, are μ z = −0.2613 ea 0 for the dipole moment and α ¯ = 47.70, Δ α = 18.41 e 2 a 0 2 E h - 1 for the mean and anisotropy of the polarizability. For the first and second hyperpolarizability we report β ¯ = 45.86 e 3 a 0 3 E h - 2 and γ ¯ = 20 682 e 4 a 0 4 E h - 3 . Very important electron correlation effects were found for the first hyperpolarizability. A comparison with the properties of ethene brings forth the empirical relationships α ¯ (H 2Si CH 2) ≈ 1.7 × α ¯ (H 2C CH 2) and γ ¯ (H 2Si CH 2) ≈ 3 × γ ¯ (H 2C CH 2).

Atomic electric-dipole moments from Higgs-boson-mediated interactions

Electric-dipole moments of paramagnetic atoms can provide important insights into a unique kind of CP violation involving Higgs-boson-mediated interactions between leptons and hadrons. We report a limit for the coupling constant of such an interaction by combining the results of our relativistic many-body calculations and experimental data on the electric-dipole moment of atomic thallium. The importance of electron correlation in determining this limit is highlighted and its significance in the context of particle physics and cosmology is pointed out.

Intensity Inversion between Main and Satellite Lines in Atomic Photoionization

The 1s photoionization of atomic Li was studied by photoelectron spectroscopy in the photon energy region between 85 and 140 eV for the ground state and the three lowest excited configurations Li(*) 1s(2)nl, nl=2p, 3s, 3p. The importance of electron correlations was investigated by comparing the multielectron transitions, so-called shake-up and conjugate shake-up satellites, and the direct process, so-called main lines. The relative intensity of the satellites increases with the level of initial excitation of the Li atom. The shake-up process dominates for states with an n=3 valence electron and the satellites become stronger than the main lines. This spectacular effect can be explained by the spatial overlap of the initial and final state wave functions. Surprisingly, the spatial overlap affects shake-up and conjugate shake-up lines in the same way.

Lifetimes of the metastableD3∕2,5∕22states inCa+,Sr+, andBa+

Atomic properties involving the low-lying excited $^{2}D_{3∕2,5∕2}$ states in alkaline-earth-metal ions are of current interest in many different applications ranging from tests of physics beyond the standard model to astrophysics. We have used the relativistic coupled-cluster theory to perform accurate calculations of the lifetimes of the lowest excited $^{2}D_{3∕2}$ and $^{2}D_{5∕2}$ states in singly ionized calcium, strontium, and barium. The importance of electron correlation in these calculations is emphasized. Our results are compared with the available experimental and theoretical data.

Actinide Chemistry in Solution, Quantum Chemical Methods and Models

Theoretical modeling of actinide complexes requires access to structural parameters, information on electronic, vibration and rotation energy levels and thermodynamics properties. This article presents a critical review of theoretical studies of actinide chemistry in gas-phase and in solution and a comparison with experimental data in order to assess the applicability and accuracy by which various electronic structure theories can predict the required quantities. The quality of the basis set, the importance of electron correlation, the description of solute-solvent interactions is discussed in some detail.

The influence of electron correlation and relativistic effects on the valence shellphotoelectron spectrum of iodothiophene

The valence shell photoelectron spectrum of 2-iodothiophene (2-I-Th) hasbeen studied experimentally and calculations have been carried out tocharacterize the main bands due to single-hole ionized states for both2- and 3-iodothiophene. The importance of electron correlation in theformation of satellite states has also been assessed. The vertical ionizationenergies and spectral intensities of the entire valence shell photoelectronspectrum have been computed using the third-order algebraic–diagrammaticconstruction approximation for the one-particle Green function. These theoreticalpredictions have allowed assignments to be proposed for all the prominentstructure observed in the experimental spectra, and have highlightedthe breakdown of the molecular orbital model of ionization for the π1-orbital.By comparing the present theoretical and experimental results for iodothiophenewith similar data for chloro-and bromothiophene, it has been found that thebinding energies and intensities of the satellites associated with the π1-orbitalform a pattern which is common to all three monohalothiophenes. RelativisticHartree–Fock calculations have been carried out to investigate the importance ofsuch effects on the valence shell ionization of iodothiophene. Synchrotron radiationhas been used to measure photoelectron angular distributions and branching ratios of2-I-Th. The spectral behaviour of the asymmetry parameters has provided anexperimental means of distinguishing photoelectron bands due to σ-orbitals from thoseassociated with π-orbitals.A high-resolution photoelectron spectrum of the outer valence shell of 2-I-Th hasbeen recorded using HeI radiation, and vibrational structure has been observed andanalysed in the X̃ 2A′′, Ã 2A′′, B̃ 2A′ and C̃ 2A′′state bands.

Metal-insulator transition of ferromagnetic ordered double perovskites: (Sr1−yCay)2FeReO6

Temperature- and composition-control metal-insulator (M-I) transitions have been investigated for ordered double perovskites, $({\mathrm{Sr}}_{1\ensuremath{-}y}{\mathrm{Ca}}_{y}{)}_{2}{\mathrm{FeReO}}_{6}.$ ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ $(y=1)$ shows the thermally induced M-I transition associated with the lattice-structural transition around 150 K. The alloyed system undergoes the M-I transition in the almost fully spin-polarized ground state around $y=0.4$ with minimal enhancement of the electronic specific-heat coefficient but with a large energy-scale $(\ensuremath{\sim}1 \mathrm{eV})$ change of the optical conductivity spectrum. These results indicate the importance of electron correlation, in particular the orbital correlation of Re ${t}_{2g}$ electrons, for the ferromagnetic M-I transition.

Resonant photoionization cross sections and branching ratios for atomic oxygen

Total and partial photoionization cross sections for the ejection of either a $2p$ or a $2s$ electron from the ground ${2s}^{2}{2p}^{4}{}^{3}P$ state of atomic oxygen have been investigated using the R-matrix method. Cross sections are dominated by Rydberg series of autoionizing resonances converging to several ionic states. The autoionizing states are analyzed and identified using a procedure of eigenphase gradients. Comparison of our results with available previous calculations and experiments is shown to lead to some interesting conclusions regarding the importance of electron correlation and normalization of the measured relative cross sections. Our calculation removed long-standing discrepancy between theory and experiment regarding the shape of photoionization cross section near the ${}^{2}{P}^{o}$ threshold. The calculated branching ratios for the production of ${}^{4}{S}^{o},{}^{2}{D}^{o},$ and ${}^{2}{P}^{o}$ states at 584 and $304 \mathrm{\AA{}}\mathrm{}$ show good agreement with experiments.

Gauge-Including Atomic Orbital Proton Chemical Shifts of Strong Hydrogen Bonds: The Importance of Electron Correlation

The effects of electron correlation are often of little importance in theoretical 1H NMR chemical shift calculations. Indeed, the differences between uncorrelated and correlated values are typically ca. 0.2 ppm or less for organic compounds. Here we demonstrate a very important case where this assumption breaks down; protons involved in strong hydrogen bonds. We found that the isotropic shifts calculated with the gauge-including atomic orbital (GIAO) approach at the RHF level overestimate the corresponding MP2 values by well over 1 ppm and commonly by 2 ppm. This is true for minimum energy geometries as well as for the transition states for proton transfer. In contrast, electron correlation effects are an order of magnitude smaller for the non-hydrogen-bonding protons in the structures we studied. The systems treated theoretically were FHF-, N2H7+, H3O2-, the enol of 2,4-pentanedione, the monoanion of cis-maleic acid, and the monoanion of dimethylmalonic acid. Geometries were calculated at either the MP2/...

Experimental and theoretical cross sections for photoionization of metastable and atoms near threshold

Using high-resolution laser photoelectron spectrometry we have determined absolute cross sections and the electron angular distribution parameter for one-photon ionization of metastable and atoms to the resolved ion states near threshold -244 nm). For comparison with the experimental data we have carried out many-electron calculations of the relevant partial cross sections and parameters over the photoelectron energy range (0-6 eV). We have used a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF), designed to take into account the important electron correlation effects which are found to be essential for obtaining satisfactory agreement between the theoretical and experimental results. Including previously reported cross sections for metastable and atoms we present a discussion of the trends which are observed in the different energy-dependent cross sections as a function of the atomic number.

Correlated calculations of molecular dynamic polarizabilities

Frequency-dependent molecular polarizabilities of several molecules N2, CO, CO2, Cl2, C2H2, COS, and CS2 are calculated by the equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method. The EOM-CCSD CI-like, linear and quadratic methods for dynamic second-order properties are presented. The importance of electron correlation, the quadratic contribution, and orbital relaxation effects are assessed. London dispersion coefficients are calculated by numerical integration of the EOM-CCSD polarizabilities.

The role of final-state correlation in single ionization of helium

Ionization of atomic helium by a fully stripped heavy-particle projectile nucleus is considered. The importance of electron correlation in the final-state bound - continuum wavefunction of this two-electron system is examined. We obtain an approximate solution of the two-electron Schrödinger equation for this particular configuration. The method used is a variational-perturbative technique, which is extensively used in approaches to the alternative distinct transitions (bound - bound and continuum - continuum) of this two-electron system. We then introduce an effective charge which we show greatly improves this description. We apply the wavefunctions derived to evaluate total and singly differential cross sections for single ionization of helium by various differently charged projectiles. Finally, we compare results with experiment.

Experimental and theoretical cross sections for photoionization of metastable Xe $^*$ $(6s\, {^3\! P}_2,\, {^3\! P}_0)$ atoms near threshold

Using high resolution laser photoelectron spectrometry we have determined absolute cross sections σJ0J1 and the electron angular distribution parameter for one photon ionization of metastable Xe*(6s3PJ0, J0 = 2, 0) atoms to the resolved Xe+ (2PJ1, J1 = 3/2, 1/2) ion states at several wavelengths near threshold. For comparison with the present and future experimental data we have calculated partial cross sections and s-parameters for photoionization of Xe*(6s3PJ0, J0 = 2, 0) and of the analogous alkali atom Cs(6s) over the photoelectron energy range (0–5) eV. We have used both a term-dependent Pauli-Fock (PF) approach and a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF). Through the PF method we include relativistic effects on the atomic orbitals; the CIPF method was designed to take into account the important electron correlation effects which are found to be essential for obtaining good agreement between the theoretical and experimental results.

Electron‐Ion Recombination of Neutral Iron

The total and state-specific electron-ion recombination rate coefficients are obtained for Fe I. The calculations are carried out using a new ab initio method that incorporates both the radiative and the dielectronic recombination processes in an unified and self-consistent manner. The computations employ the close coupling approximation and the R-matrix method from atomic collision theory. A 52 state close coupling eigenfunction expansion dominated by the states of the ground 3d64s and excited 3d7, 3d64p, 3d54s2, and 3d54s4p configurations of Fe II are used in the present calculations. The important electron correlation and radiation damping effects are included via explicit coupling of autoionization and radiative channels. This is the first detailed atomic calculation for the recombination rates for Fe I. The present rates are considerably higher than the radiative recombination rates being used currently in the low-temperature region, T ≤ 104 K, whereas they are about 4 times lower than those given by the Burgess general formula for dielectronic recombination at higher temperatures. The implications of the new recombination rate coefficients and photoionization cross sections for Fe I on the ionization structure of iron in the cold neutral interstellar medium are studied. It is found that the ratio of Fe II to Fe I obtained with the new atomic data increases by a factor of about 3-30 over previous calculations.

On the magnetic coupling in NiO

The results are reported of ab initio calculations on the magnetic ordering in NiO, a prototype of the antiferromagnetic insulator. By analyzing wave functions for different cluster models, information is obtained about the physical effects determining the sign and the magnitude of the magnetic coupling parameter J. The role of the edge oxygens, surrounding the essential unit (Ni2O), is found to be quantitatively important but purely environmental in contrast to the role of the bridging oxygen. Furthermore, the importance of electron correlation and the usefulness of pseudopotentials in the calculations is investigated. The final result for J compares reasonably with experiment (about 50%), and possible sources for the remaining discrepancies are discussed.

Electronic state of the organic salt (DI-DCNQI)2Ag, where DI-DCNQI is 2,5-diiodo-N,N'-dicyanoquinonediimine.

${(\mathrm{D}\mathrm{I}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})}_{2}$Ag where DI-DCNQI is 2,5-diiodo-$N$, ${N}^{\ensuremath{'}}$-dicyanoquinonediimine has been investigated by electrical resistance, magnetic susceptibility, and $^{1}\mathrm{H}$ NMR measurements, and found to be nonmetallic with a charge gap of \ensuremath{\sim}490 K and with spin susceptibility showing localization of the $\ensuremath{\pi}$-electron spins. Antiferromagnetic ordering is evidenced at a low temperature. This is a first case of the electron antiferromagnet among the DCNQI-metal family in which the $\ensuremath{\pi}$ electrons on the DCNQI columns have an antiferromagnetic state. The nature of a Mott insulator demonstrates the importance of electronic correlation in the $\ensuremath{\pi}$ electrons in the DCNQI systems.

Experimental and theoretical cross sections for photoionization of metastable atoms near threshold

Using high-resolution laser photoelectron spectrometry we have determined for the first time absolute cross sections and the electron angular distribution parameter for one-photon ionization of metastable atoms to the resolved ion states near threshold as well as the -parameter for metastable atoms at , 257.25 nm. For comparison with present and future experimental data we have calculated partial cross sections and -parameters for photoionization of , and of the analogous alkali atom over the photoelectron energy range 0 - 10 eV. We have used both a term-dependent Hartree - Fock approach and a configuration-interaction method involving Pauli - Fock atomic orbitals (CIPF); the latter was designed to take into account the important electron correlation effects which are found to be essential for obtaining good agreement between the theoretical and experimental results. .

Rb$\mathsf{_1}$C$\mathsf{_{60}}$ under Pressure: an NMR and ESR Study

We report the results of magnetic measurements on the orthorhombic alkalimetal fulleride Rb 1 C 60 . Measurements of the NMR relaxation T 1 and ESR spin susceptibility under pressure (P ≤ 5 kbar) provide clear evidence for the importance of electron correlations in this phase at variance with superconducting phase 3 fullerides. We show that spin fluctuations (ferromagnetic) persisting up to room temperature order antiferromagnetically at low temperature and give rise to long range order below 15 K. These results support the band structure calculation describing this polymerised phase by a three-dimensional model. The magnetic ground state is suppressed by a pressure exceeding 6 kbar and gives rise to a conducting phase at 12 kbar whose susceptibility is still possibly enhanced by magnetic fluctuations.

Nature of the electronic states in the layered perovskite noncuprate superconductor Sr2RuO4.

Near-edge x-ray-absorption and photoemission spectra of the noncuprate superconductor ${\mathrm{Sr}}_{2}$Ru${\mathrm{O}}_{4}$ are presented. Holes close to the Fermi level are determined by Ru $4{d}_{\mathrm{xy}}$, $4{d}_{\mathrm{xz}}$, and $4{d}_{\mathrm{yz}}$ antibonding states. The density of the occupied states at the Fermi level derived from photoemission is higher than in the cuprates and these states have predominantly Ru $4d$ character. Angle-resolved photoemission yields a width of the occupied part of the conduction bands that is reduced by a factor of 2 in comparison to band-structure calculations. The importance of electron correlations is also evident in the appearance of screened final states in the Ru $3d$ core-level spectrum.

Structure and Stability of Molecular Carbon: Importance of Electron Correlation

Structure and Stability of Molecular Carbon: Importance of Electron Correlation