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Electron Correlation (electron + correlation)
Terms modified by Electron Correlation Selected AbstractsChemInform Abstract: Quantitative Analysis of Electron Correlation in M2Po (M: Cu, Ag, Au) SystemsCHEMINFORM, Issue 35 2009Q.-M. Surong Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Comment on "The Water-Exchange Mechanism of the [UO2(OH2)5]2+ Ion Revisited: The Importance of a Proper Treatment of Electron Correlation" [F.CHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2007Fulfilled satisfactorily: As a reply to the statements of F. P. Rotzinger [Chem. Eur. J., 2007, 13, 800] we demonstrate that the MP2 method can be applied to uranyl(VI) complexes because dynamic electron correlation effects dominate and static electron correlation of the near-degeneracy type is absent. [source] Electron correlation: The many-body problem at the heart of chemistryJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2007David P. Tew Abstract The physical interactions among electrons and nuclei, responsible for the chemistry of atoms and molecules, is well described by quantum mechanics and chemistry is therefore fully described by the solutions of the Schrödinger equation. In all but the simplest systems we must be content with approximate solutions, the principal difficulty being the treatment of the correlation between the motions of the many electrons, arising from their mutual repulsion. This article aims to provide a clear understanding of the physical concept of electron correlation and the modern methods used for its approximation. Using helium as a simple case study and beginning with an uncorrelated orbital picture of electronic motion, we first introduce Fermi correlation, arising from the symmetry requirements of the exact wave function, and then consider the Coulomb correlation arising from the mutual Coulomb repulsion between the electrons. Finally, we briefly discuss the general treatment of electron correlation in modern electronic-structure theory, focussing on the Hartree-Fock and coupled-cluster methods and addressing static and dynamical Coulomb correlation. © 2007 Wiley Periodicals, Inc. J Comput Chem 28: 1307,1320, 2007 [source] Theoretical Description of Substituent Effects in Electrophilic Aromatic Substitution ReactionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 35 2008Tobias Schwabe Abstract The ability of the current Kohn,Sham density functional theory (DFT) to compute the change of the proton affinity (PA) of phenol derivatives due to substitution is investigated. These systems can be used as models to predict reactivities in electrophilic aromatic substitution reactions. The complexity of the problem is increased systematically by introducing successively up to four substituents in five typical cases (methyl, cyano, fluorine, chlorine, and bromine). Our investigation can be regarded as representative for an important class of problems consistently encountered in the DFT modeling of organic reactions. High-level theoretical reference data from CCSD(T) and SCS-MP2 wave-function calculations are presented, and the PAs are compared to those obtained by a series of density functionals (DFs). It is shown that not all DFs are capable of quantitatively reproducing the substituent effects. These can be simply linear in the number of substituents or show more complicated patterns. Especially for halogens, some DFs even fail completely. In these cases, linearly increasing errors with the number of groups are observed. Reliable results are obtained with hybrid DFs or the even more accurate double-hybrid DF approach. The errors are attributed to the common self-interaction (over-delocalization) error in part of the DFs. Comparison with Hartree,Fock results shows that a reliable account of electron correlation is necessary to compute the PA of unsaturated and highly substituted molecules with chemical accuracy.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Theory of chemical bonds in metalloenzymes.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010Abstract Reaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theoretical grounds, together with experimental results. First of all, our previous broken-symmetry (BS) molecular orbitals (MO) calculations are reviewed to elucidate the instability of the d,-p, bond in high-valent (HV) Mn(X)O systems and the d,-p,-d, bond in HV MnOMn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: ,Mn(IV)O, and ,MnOMn,; the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theoretical results, the BS MO interaction diagrams for oxygen-radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of oxygenation reactions mechanisms by p450 and methane monooxygenase (MMO). The manganese-oxide cluster models examined are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of molecular oxygen in the total low-, intermediate and high-spin states of these clusters. The spin alignments are found directly corresponding to the spin-coupling mechanisms of oxygen-radical sites in these clusters. The BS UB3LYP calculations of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examined in comparison with a lot of accumulated experimental and theoretical results for oxygen evolution reactions in native and artificial PSII systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] Correlation studies in weakly confining quantum dot potentialsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2008Peter Kimani Abstract We investigate the electron correlation in few-electron closed-shell atomic systems and similarly in few-electron quantum dots under weak confinement. As usual we start with restricted Hartree,Fock (HF) calculations and add electron correlation in steps in a series of approximations based on the single particle Green's function approach: (i) second-order Green function (GF); (ii) 2ph -Tamm-Dancoff approximation (TDA); and (iii) an extended version thereof which introduces ground-state correlation into the TDA. Our studies exhibit similarities and differences between weakly confined quantum dots and standard atomic systems. The calculations support the application of HF, GF, and TDA techniques in the modeling of three-dimensional quantum dot systems. The observed differences emphasize the significance of confinement and electronic features unique to quantum dots, such as the increased binding of electrons with higher angular momentum and thus,compared to atomic systems,modified shell-filling sequences. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Natural bond orbital-based energy density analysis for correlated methods: Second-order Møller,Plesset perturbation and coupled-cluster singles and doublesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 8 2008Yutaka Imamura Abstract Natural bond orbital-based energy density analysis (NBO-EDA), which split energies into atomic and bonding contributions, is proposed for correlated methods such as coupled-cluster singles and doubles (CCSD) and second-order Møller,Plesset (MP2) perturbation. Applying NBO-EDA for CCSD and MP2 to ethylene and the Diels,Alder reaction, we are successful in obtaining useful knowledge regarding electron correlation of ,- and ,-type orbitals, and clarifying the difference of the reaction barriers and heat of reaction calculated by CCSD and MP2. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Tuning of electron transport through molecular bridge systems: A study of shot noiseINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2008Santanu K. Maiti Abstract We study electron transport characteristics through a single phenalenyl molecule attached with two nonsuperconducting electrodes by the use of Green's function technique. Parametric calculations are given based on the tight-binding model to characterize the electron transport through such molecular bridge system. It is observed that the electron transport properties are significantly influenced by (a) the interference effect and (b) the molecule-to-electrodes coupling strength. In this context we also describe the noise power of the current fluctuations that provides an important information about the electron correlation, which is obtained by calculating the Fano factor (F). The knowledge of this current fluctuations gives a key idea for fabrication of efficient molecular devices. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Correlated one-electron wave functionsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2005B. Weiner Abstract A one-electron theory of many-electron systems that explicitly describes electron,electron correlation may be based on the observation that antisymmetrized geminal power (AGP) states are completely characterized by the occupation numbers of their first-order reduced density operators (FORDO) and a set of canonical general spin,orbitals (CGSOs), which are in general distinct from the natural general spin,orbitals. Because the FORDO alone does not determine an AGP state, a density matrix functional may be defined. A generalization of the independent-particle Fock operator serves to express the total state energy in terms of ionization energies, kinetic energies, and potential energies of the CGSOs. A one-electron picture of chemical bonding emerges from this theory. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Explicitly correlated SCF study of anharmonic vibrations in (H2O)2INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2002Donald D. Shillady Abstract Modeling solvation in high-pressure liquid chromatography (HPLC) requires calculation of anharmonic vibrational frequencies of solvent clusters for a statistical partition function. An efficient computational method that includes electron correlation is highly desirable for large clusters. A modified version of the "soft Coulomb hole" method of Chakravorty and Clementi has recently been implemented in a Gaussian-lobe-orbital (GLO) program (PCLOBE) to include explicit electron,electron correlation in molecules. The soft Coulomb hole is based on a modified form of Coulomb's law: An algorithm has been developed to obtain the parameter "w" from a polynomial in the effective scaling of each primitive Gaussian orbital relative to the best single Gaussian of the H1s orbital. This method yields over 90% of the correlation energy for molecules of low symmetry for which the original formula of Chakravorty and Clementi does not apply. In this work, all the vibrations of the water dimer are treated anharmonically. A quartic perturbation of the harmonic vibrational modes is constrained to be equal to the exact Morse potential eigenvalue based on a three-point fit. This work evaluates the usefulness of fitting a Morse potential to a hydrogen bond vibrational mode and finds it to be slightly better than using MP2 vibrational analysis for this important dimer. A three-point estimate of the depth, De, of a Morse potential leads to a correction formula for anharmonicity in terms of the perturbed harmonic frequency: When scaled by 0.9141, the harmonic Morse method leads to essentially the same results as scaling the BPW91 local density method by 0.9827. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] Temperature effects on the UV,Vis electronic spectrum of trans-stilbeneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001S. P. Kwasniewski Abstract The ultraviolet (UV),Visible absorption spectrum of trans-stilbene (tS) is computed at different temperatures by coupling molecular dynamics (MD) simulations with the classical MM3 force field to ZINDO/S-CIS calculations of vertical excitation energies and transition dipole moments. The selection of a large number of structures along the MD trajectories enables a consistent treatment of temperature effects in the vacuum, whereas the ZINDO/S-CIS calculations permit a reliable treatment of electron correlation and relaxation, taking account of multistate interactions in the final state. Thermal motions are found to alter very differently the width and shape of bands. Structural alterations such as the stretching and the torsion of the vinyl single and double bonds very strongly influence the appearance of the first valence state, pertaining to the highest occupied and lowest unoccupied molecular orbital (HOMO,LUMO) transition. At temperatures less than 400 K, these are found to yield a merely Gaussian and very pronounced thermal broadening of the related band (A), up to nearly 30 nm, together with a minor blue shift of its maximum ,max. In contrast, a red shift by several nanometers occurs due to thermal motions for the remaining three valence bands. As can be expected, the broadening intensifies at higher temperatures, and for the A-band, becomes markedly asymmetric when T exceeds 400 K. The combination of MD(MM3) and ZINDO/S-CIS computations enables also consistent calculations of hot bands, which are forbidden by symmetry at 0 K. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Half metallic properties of LaSrVMoO6JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2010Weiyu Song Abstract The recent synthesized LaSrVMoO6 was speculated to be compensated half metal, i.e., half metal with zero magnetic moment. Based on the experimental structure, our first principles study indicates that it is ferrimagnetic and half metallic with the magnetic moment 2.0 ,B when the electron correlation of Mo 4d electrons is larger than 2.72 eV. This indicates the strong electron correlation effect of Mo 4d electrons. Nonetheless, the obtained large magnetic moment (2.0 ,B) contradicts with the experimental observed nearly zero magnetic moment. Although the large antisite defects of the experimental sample might be the reason to reduce the saturated magnetic moment, further physical insights need to be investigated. The spin-orbit coupling effect has minor effect on the studied properties. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Electron localizability indicators ELI and ELIA: The case of highly correlated wavefunctions for the argon atomJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2008Viktor Bezugly Abstract Electron localizability indicators based on the same-spin electron pair density and the opposite-spin electron pair density are studied for correlated wavefunctions of the argon atom. Different basis sets and reference spaces are used for the multireference configuration interaction method following the complete active space calculations aiming at the understanding of the effect of local electron correlation when approaching the exact wavefunction. The populations of the three atomic shells of Ar atom in real space are calculated for each case. © 2007 Wiley Periodicals, Inc. J Comput Chem 29: 1198,1207, 2008 [source] Electron correlation: The many-body problem at the heart of chemistryJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2007David P. Tew Abstract The physical interactions among electrons and nuclei, responsible for the chemistry of atoms and molecules, is well described by quantum mechanics and chemistry is therefore fully described by the solutions of the Schrödinger equation. In all but the simplest systems we must be content with approximate solutions, the principal difficulty being the treatment of the correlation between the motions of the many electrons, arising from their mutual repulsion. This article aims to provide a clear understanding of the physical concept of electron correlation and the modern methods used for its approximation. Using helium as a simple case study and beginning with an uncorrelated orbital picture of electronic motion, we first introduce Fermi correlation, arising from the symmetry requirements of the exact wave function, and then consider the Coulomb correlation arising from the mutual Coulomb repulsion between the electrons. Finally, we briefly discuss the general treatment of electron correlation in modern electronic-structure theory, focussing on the Hartree-Fock and coupled-cluster methods and addressing static and dynamical Coulomb correlation. © 2007 Wiley Periodicals, Inc. J Comput Chem 28: 1307,1320, 2007 [source] A CSOV study of the difference between HF and DFT intermolecular interaction energy values: The importance of the charge transfer contributionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2005Jean-Philip Piquemal Abstract Intermolecular interaction energy decompositions using the Constrained Space Orbital Variation (CSOV) method are carried out at the Hartree,Fock level on the one hand and using DFT with usual GGA functionals on the other for a number of model complexes to analyze the role of electron correlation in the intermolecular stabilization energy. In addition to the overall stabilization, the results provide information on the variation, with respect to the computational level, of the different contributions to the interaction energy. The complexes studied are the water linear dimer, the N -methylformamide dimer, the nucleic acid base pairs, the benzene,methane and benzene-N2 van der Waals complexes, [Cu+ -(ImH)3]2, where "ImH" stands for the Imidazole ligand, and ImH-Zn++. The variation of the frozen core energy (the sum of the intermolecular electrostatic energy and the Pauli repulsion energy) calculated from the unperturbed orbitals of the interacting entities indicates that the intramolecular correlation contributions can be stabilizing as well as destabilizing, and that general trends can be derived from the results obtained using usual density functionals. The most important difference between the values obtained from HF and DFT computations concerns the charge transfer contribution, which, in most cases, undergoes the largest increase. The physical meaning of these results is discussed. The present work gives reference calculations that might be used to parametrize new correlated molecular mechanics potentials. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1052,1062, 2005 [source] Accurate prediction of proton chemical shifts.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2001Abstract Forty-five proton chemical shifts in 14 aromatic molecules have been calculated at several levels of theory: Hartree,Fock and density functional theory with several different basis sets, and also second-order Møller,Plesset (MP2) theory. To obtain consistent experimental data, the NMR spectra were remeasured on a 500 MHz spectrometer in CDCl3 solution. A set of 10 molecules without strong electron correlation effects was selected as the parametrization set. The calculated chemical shifts (relative to benzene) of 29 different protons in this set correlate very well with the experiment, and even better after linear regression. For this set, all methods perform roughly equally. The best agreement without linear regression is given by the B3LYP/TZVP method (rms deviation 0.060 ppm), although the best linear fit of the calculated shifts to experimental values is obtained for B3LYP/6-311++G**, with an rms deviation of only 0.037 ppm. Somewhat larger deviations were obtained for the second test set of 4 more difficult molecules: nitrobenzene, azulene, salicylaldehyde, and o -nitroaniline, characterized by strong electron correlation or resonance-assisted intramolecular hydrogen bonding. The results show that it is possible, at a reasonable cost, to calculate relative proton shieldings in a similar chemical environment to high accuracy. Our ultimate goal is to use calculated proton shifts to obtain constraints for local conformations in proteins; this requires a predictive accuracy of 0.1,0.2 ppm. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1887,1895, 2001 [source] A post-SCF quantum chemistry study on local minima of 8-oxo-guanine stacked with all four nucleic acid bases in B-DNA conformationsJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 4 2007Piotr Cysewski The post SCF MP2/6-31G*(d=0.25) method was applied to obtain potential energy surface of 8-oxoguanine stacked with all four canonical DNA bases. The spatial neighbourhood was scanned of stacked complexes found in the native B-DNA. The presented results suggest that the hydroxyl radical modification of guanine at C8 position has significant impact on structural, energetic, orbital and electrostatic properties of stacked complexes with canonical DNA bases. The pair stabilization energy, including electron correlation terms, suggests that the 5,-A/GA-3, pair is the most stable among all of the studied complexes. The 8-oxo-guanine has been found as a source of significant changes in electroaccepting properties compared to stacked pairs formed by canonical guanine since both electron affinities and localization of HOMO orbital were altered. However, electro-donation abilities are not modified after replacement of guanine with 8-oxo-guanine irrespectively on the context of B-DNA bases. [source] Comment on "The Water-Exchange Mechanism of the [UO2(OH2)5]2+ Ion Revisited: The Importance of a Proper Treatment of Electron Correlation" [F.CHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2007Fulfilled satisfactorily: As a reply to the statements of F. P. Rotzinger [Chem. Eur. J., 2007, 13, 800] we demonstrate that the MP2 method can be applied to uranyl(VI) complexes because dynamic electron correlation effects dominate and static electron correlation of the near-degeneracy type is absent. [source] The Origin of Aromaticity: Important Role of the Sigma Framework in BenzeneCHEMPHYSCHEM, Issue 9 2004Borislav Kova Abstract The physical nature of aromaticity is addressed at a high ab initio level. It is conclusively shown that the extrinsic aromatic stabilization energy of benzene E(ease)B, estimated relative to its linear polyene counterpart(s), is very well-reproduced at the Hartree,Fock (HF) level. This is a consequence of the fact that the contributions arising from the zero-point vibrational energy (ZPVE) and electron correlation are rather small. More specifically, they yield together 2.0 kcal,mol,1to the destabilization of benzene. A careful scrutiny of the HF energies by virial theorem shows further that the kinetic energies of the , and , electrons E(T) and E(T) are strictly additive in the gauge linear zig-zag polyenes, which also holds for their sum Et(T)HF. This finding has the important corollary that E(ease)Bis little dependent on the choice of the homodesmic reactions involving zig-zag polyenes. A detailed physical analysis of the ,- and ,-electron contributions to extrinsic aromaticity requires explicit introduction of the potential energy terms Vne, Vee, and Vnn, which signify Coulomb interactions between the electrons and the nuclei. The Veeterm involves repulsive interaction V between the , and , electrons, which cannot be unequivocally resolved into , and , contributions. The same holds for the Vnnenergy, which implicitly depends on the electron density distribution via the Born,Oppenheimer (BO) potential energy surface. Several possibilities for partitioning V and Vnnterms into , and , components are examined. It is argued that the stockholder principle is the most realistic, which strongly indicates that E(ease)Bis a result of favorable , -framework interactions. In contrast, the ,-electron framework prefers the open-chain linear polyenes. [source] Principal component analysis of the effects of wavefunction modification on the electrostatic potential of indoleINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2005Maíra A. Carvalho Abstract The molecular electrostatic potential (MEP) of the indole molecule was calculated in a three-dimensional grid in which the molecule was centered at the origin. To evaluate the dependence of MEP on the type of calculation, semiempirical, ab initio, and density functional theory methods with different basis sets were employed. The data matrix generated by these calculations was analyzed by principal component analysis (PCA). The appearance of outliers and the effect of wavefunction modifications such as the introduction of electron correlations and diffuse functions were highlighted by the use of PCA. The spatial localization of such effects around the molecule was possible from the loadings values associated with the graphical analysis of the grid points. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Analysis of difference two-electron density matrix between two states of magnetic moleculesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001D. Yamaki Abstract Previously, a method based on the two-electron density matrix (2-DM) has been used to obtain chemical interpretations with electron correlations. In the method, a 2-DM is expanded by its natural geminals. These natural geminals are visualized to obtain their chemical interpretations. An electronic state is understood as the set of geminals that have various correlations of two electrons. In this work, a extension of previous 2-DM-based method is described to compare two states of a system. In this method, the difference 2-DM between the two states is used instead of 2-DM itself. The applications of the method to trans-butadiene and trimethylene methane are shown. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Influence of electron correlations on strong field ionization of calciumLASER PHYSICS LETTERS, Issue 1 2004E. Charron Abstract Non-perturbative time-dependent calculations of single and double ionization of a one-dimensional model of atomic calcium by short and intense laser pulses were performed at various wavelengths. The comparison of the probabilities calculated within a two-active electron (TAE) approach with those obtained using a single-active electron (SAE) approximation clearly demonstrates the crucial role played by the electronic correlation and by doubly excited states within this model, even for the formation of Ca+. Experimental and calculated energy spectra of the singlet states of Ca. The experimental values (b) are taken from [20], and the calculated values (a) on the left and (c) on the right correspond to the present SAE and TAE models respectively. (© 2004 by HMS Consultants. Inc. Published exclusively by WILEY-VCH Verlag GmbH & Co.KGaA) [source] | ||||||||||