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Configuration Interaction (configuration + interaction)

Distribution by Scientific Domains

Chemistry	92%
Physics and Astronomy	7%

Kinds of Configuration Interaction

multireference configuration interaction

Terms modified by Configuration Interaction

configuration interaction method

Selected Abstracts

Origin of the Paramagnetic Properties of the Mixed-Valence Polyoxometalate [GeV14O40]8, Reduced by Two Electrons: Wave Function Theory and Model Hamiltonian Calculations

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 34 2009
N. Suaud
Abstract The aim of the work is to give an explanation of the magnetic properties of a mixed-valence [GeV14O40]8, polyoxometalate reduced by two electrons, which, in contrast to what happens in other two-electron-reduced polyoxometalates, does not show any magnetic coupling between the two unpaired electrons. For this purpose, a quantitative evaluation of the microscopic electronic parameters (electron transfer, magnetic coupling, magnetic orbital energy, and Coulomb repulsion) of the mixed-valence polyoxometalate cluster is performed. The parameters are extracted from valence-spectroscopy large configuration interaction (CI) calculations on embedded fragments. Then, these parameters are used in an extended t - J model Hamiltonian suited to model the properties of the whole anion. The analysis of the wave functions of the lowest singlet and triplet states and of the microscopic parameters emphasizes that the electron delocalization in this mixed-valence cluster is such that each unpaired electron is almost trapped in a different half of the polyoxovanadate, thus disabling any exchange interaction between them.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Kinetic isotope effects for the H2 + C2H , C2H2 + H reaction based on the ab initio calculations and a global potential energy surface

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2010
Liping Ju
In the present paper, kinetic isotope effects of the title reaction are studied with canonical variational transition state theory on the modified Wang Bowman (MWB) potential energy surface (PES) (Chem Phys Lett 2005, 409, 249) and the ab initio calculations at the quadratic configuration interaction (QCISD (T, full))/aug-cc-pVTZ//QCISD (full)/cc-pVTZ level. The calculated rate constants for the isotopic variants of this title reaction on the MWB PES have good agreement with those of the present ab initio calculations over the temperature range of 20,5000 K for the forward reactions and 800,5000 K for the reverse reactions, respectively. In particular, the forward rate constants for the title reaction and its isotopically substituted reactions have negative temperature dependences at about 40 K. Rate expressions are presented for all the studied reactions. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 289,298, 2010 [source]

Nonrelativistic CI calculations for B+, B, and B, ground states

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010
César X. Almora-Díaz
Abstract State of the art configuration interaction (CI) techniques are used to obtain the best possible nonrelativistic CI results for B+, B, and B, ground states using energy-optimized basis sets of 252, 294, and 294 radial Slater-type functions, respectively. For positive boron, E(B+) = ,24.348861 + ,Ebie = ,24.348883(1) a.u.(B) with a basis set incompleteness error ,Ebie = ,0.000022(1), in good agreement with the latest exponentially correlated Gaussian (ECG) result of ,24.348883 a.u.(B) of Komasa et al. (Phys Rev A, 2002, 65, 042507). For neutral B, E(B) = ,24.653837 ,0.000024(2) = ,24.653861(2), which is the most accurate ab initio estimate and lies slightly below a recent (not fully optimized) ECG result of ,24.653840 a.u.(B) of Bubin et al (J Chem Phys, 2009, 131, 044128). For negative boron, E(B,) = ,24.664014 ,0.000024(2) to which an energy error of ,0.000001 must be added to give ,24.664039(2), which is the first fully correlated ab initio result. Comparison with experimental values of ionization energy and electron affinity must await the results of corresponding relativistic calculations, in progress. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

Orbital invariance issue in multireference methods

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2010
Liguo Kong
Abstract The orbital invariance problem is analyzed from the tensor theory point of view, with an emphasis on multireference coupled cluster methods. Using the transformation properties of second-quantized operators, we discuss the orbital invariance properties of various methods by examining the tensor properties of the residual equations. A simple self-consistency-checking algorithm is proposed. We first establish the orbital invariance properties for the Hartree,Fock, single reference configuration interaction, single reference coupled cluster, complete-active-space self-consistent-field, and multireference configuration interaction methods, and then discuss the invariance properties of the complete-active-space coupled cluster and CCSDt methods. Finally, we demonstrate theoretically the lack of orbital invariance for Jeziorski,Monkhorst ansatz based methods. It appears necessary to modify the ansatz to achieve orbital invariance, and internal contraction serves as one possible solution. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

Theoretical study on the influence of ancillary ligand on the spectroscopic properties and electronic structures of phosphorescent Pt(II) complexes

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2010
Min Zhang
Abstract The geometries, energies, and electronic properties of a series of phosphorescent Pt(II) complexes including FPt, CFPt, COFPt, and NFPt have been characterized within density functional theory DFT calculations which can reproduce and rationalize experimental results. The properties of excited-states of the Pt(II) complexes were characterized by configuration interaction with singles (CIS) method. The ground- and excited-state geometries were optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ levels, respectively. In addition, we also have performed a triplet UB3LYP optimization for complex FPt and compared it with CIS method in the emission properties. The datum (562.52 nm) of emission wavelength for complex FPt, which were computed based on the triplet UB3LYP optimization excited-state geometry, is not agreement with the experiment value (500 nm). The absorption and phosphorescence wavelengths were computed based on the optimized ground- and excited-state geometries, respectively, by the time-dependent density functional theory (TD-DFT) methods. The results revealed that the nature of the substituent at the phenylpyridine ligand can influence the distributions of HOMO and LUMO and their energies. Moreover, the auxiliary ligand pyridyltetrazole can make the molecular structure present a solid geometry. In addition, the charge transport quality has been estimated approximately by the predicted reorganization energy (,). Our result also indicates that the substitute groups and different auxiliary ligand not only change the nature of transition but also affect the rate and balance of charge transfer. By summarizing the results, we can conclude that the NFPt is good OLED materials with a solid geometry and a balanced charge transfer rate. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

Electron invariants and excited state structural analysis for electronic transitions within CIS, RPA, and TDDFT models

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2010
A. V. Luzanov
Abstract We revisit the interpretative scheme (Luzanov et al., Theor Exp Chem 1974, 10, 354) of singly excited configuration interaction (CIS) model given earlier at semiempirical level. Detailed computations and spectral (natural orbital) treatment of the CIS density matrices of various types are presented. The corresponding hole-particle densities and related excitation localization indices are described. All the quantities are extended to the excited states calculated in the random phase approximation and closely related time-dependent density functional theory (TDDFT). The localization indices and charge transfer numbers which are invoked to describe interfragment interactions provide a basis for our scheme which is referred to as the excited state structural analysis for electronic transitions. The proposed analysis is exemplified by various moderate and large-size conjugated molecules treated within ab initio TDDFT and the Parizer,Parr,Pople approximation. Finally, we propose a possible generalization to the electronic transitions between CIS-like states followed by applications to singlet organic biradicals treated within the ,-electron spin-flip CIS. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

Molecular one-electron properties using the multireference Hartree,Fock CI method

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2008
A. M. C. Sobrinho
Abstract The recently introduced multireference Hartree,Fock configuration interaction (MRHFCI) method has been applied to the calculation of the dipole moment of the LiH, BH, FH, CO, and H2O molecules. The results obtained indicate that MRHFCI wave functions, much more compact but of the same quality of the orthogonal full CI ones, can provide better dipole moments than the corresponding full CI wave functions. The value of the dipole moments obtained with the MRHFCI wave functions is quite insensitive to the choice of the HF references but the same is not true for the electronic energy. Therefore, further studies are necessary to develop a criterion for selecting a set of HF references which could provide the best values of energy and dipole moment. Also, it would be important to verify if other one-electron properties can be computed with the same degree of accuracy obtained for the dipole moments. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

Spin-optimized resonating Hartree-Fock configuration interaction

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2007
Ryo Takeda
Abstract The resonating Hartree-Fock Configuration Interaction (Res HF-CI) method is an efficient tool to investigate complicated strongly correlated systems such as ion-radical systems. In this method, we explore several spin-unrestricted Hartree-Fock (UHF) solutions that are energetically low-lying. However it is difficult to choose the symmetry-broken references appropriately as the site increases. In this study, we present the spin-optimized procedure, which is based on the Löwdin spin-projection method, for the Res HF-CI theory, denoted as SO Res-HF CI. We apply this SO Res-HF CI method to depict the potential curves of typical polyradical systems and compared the computational results using complete-active-space (CAS) CI based on UHF natural orbital (UNO), spin-projected UHF, and the previous version of Res HF-CI. We discuss the relation between computational results and the electronic configurations that are important to cover the electron correlation effects for each system. Further, we apply SO Res-HF CI method for the simple organic radical. In addition, we extend this scheme to the GHF case, and show that the use of GHF as a seed of SO Res-HF CI is desirable for the spin-frustrated systems. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source]

Variational CI techniques for computing dispersion constants

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2005
Gian Luigi BendazzoliArticle first published online: 4 APR 200
Abstract We describe the computation of dispersion constants using variational subspace methods to solve the perturbation theory equations in the tensor product space of the interacting molecules, treated separately at a (full) configuration interaction (CI) level. These new techniques are more accurate than the numerical quadrature of the Casimir Polder integral at the expense of a small additional computational cost. We also show how to compute the norm of the residual of the perturbative solution in the tensor product space to check convergence. The methods are tested on LiH and BH. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source]

Ground states of BeC and MgC: A comparative multireference Brillouin,Wigner coupled cluster and configuration interaction study,

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005
Vasilios I. Teberekidis
Abstract The competing X3,, and 5,, states of the experimentally unknown alkaline,earth metal carbides BeC and MgC are examined with the multireference Brillouin,Wigner coupled cluster method restricted to single and double excitations (MRBW-CCSD). The results are compared against the traditional single-reference CCSD approach, as well as with other single and multireference methods. In both molecules, the CCSD 5,, , X3,, energy difference is underestimated, leading to an "erroneous" ground-state prediction in BeC. The MRBW-CCSD method corrects this anomalous behavior, leading to fair agreement with multireference configuration interaction (MR-CI) predictions. Our results at the highest levels of theory are extrapolated to the basis set limit, and the core/valence correlation is taken into account, leading to very accurate energetics and spectroscopic constants in both carbides. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source]

A posteriori corrections to multireference limited configuration interaction based on a Brillouin,Wigner perturbative analysis

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2002
I. Hubac
Abstract A new a posteriori correction to the method of limited configuration interaction is described that attempts to restore a linear scaling with particle number. The corrections are based on an analysis of the limited configuration interaction method in terms of the Brillouin,Wigner perturbation theory using a Lippmann,Schwinger-like equation. The new correction procedure is general and, in this work, the application to the limited multireference configuration interaction approximation is considered in some detail. An illustrative application to the rigid rotation of the diimine molecule is presented and the results are compared with those obtained by employing Davidson-like corrections and the corresponding full configuration interaction energies. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source]

Configuration interaction calculation of electronic g tensors in transition metal complexes,

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3-4 2001
Frank Neese
Abstract An algorithm for the calculation of electronic g tensors from configuration interaction (CI) wave functions and its implementation for any ground-state spin S is described. The algorithm can be used together with either single- or multireference CI wave functions but assumes a spatially nondegenerate ground state. A one-electron approximation to the spin,orbit coupling (SOC) operator is used. Presently the implementation uses the INDO/S model of Zerner and coworkers. Results are presented for a representative collection of Cu(II) complexes with N-, O-, and S-containing ligands. The calculations reproduce the trends in the experimental data well and show no systematic errors. For the test set the g values are reproduced with a standard deviation of 0.021. The method has therefore a good cost,performance ratio and is expected to be helpful in chemical and biochemical applications. Further improvements are feasible and necessary and are briefly discussed. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 104,114, 2001 [source]

Investigation of excited-state properties of fluorene,thiophene oligomers by the SAC-CI theoretical approach

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2010
Potjaman Poolmee
Abstract Excited states of fluorene-ethylenedioxythiophene (FEDOT) and fluorene- S,S -dioxide-thiophene (FTSO2) monomers and dimers were studied by the symmetry-adapted cluster (SAC)-configuration interaction (CI) method. The absorption and emission peaks observed in the experimental spectra were theoretically assigned. The first three excited states of the optimized conformers, and the conformers of several torsional angles, were computed by SAC-CI/D95(d). Accurate absorption spectra were simulated by taking the thermal average for the conformers of torsional angles from 0° to 90°. The conformers of torsional angles 0°, 15°, and 30° mainly contributed to the absorption spectra. The full width at half-maximum of the FEDOT absorption band is 0.60 eV (4839 cm,1), which agrees very well with the experimental value of 0.61 eV (4900 cm,1). The maximum absorption wavelength is located at 303 nm, which is close to those of the experimental band (327 nm). The calculated absorption spectrum of FTSO2 showed two bands in the range of 225,450 nm. This agrees very well with the available experimental spectrum of a polymer of FTSO2, where two bands are detected. The excited-state geometries were investigated by CIS/6-31G(d). These showed a quinoid-type structure which exhibited a shortening of the inter-ring distance (0.06 Å for FEDOT and 0.04 Å for FTSO2). The calculated emission energy of FEDOT is 3.43 eV, which agrees very well with the available experimental data (3.46 eV). The fwhmE is about 0.49 eV (3952 cm,1), while the experimental fwhm is 0.43 eV (3500 cm,1). For FTSO2, two bands were also found in the emission spectrum. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [source]

An ab initio potential energy surface and vibrational energy levels of ZnH2

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2010
Zheng Guo Huang
Abstract A three-dimensional potential energy surface of the electronic ground state of ZnH2 () molecule is constructed from more than 7500 ab initio points calculated at the internally contracted multireference configuration interaction with the Davidson correction (icMRCI+Q) level employing large basis sets. The calculated relative energies of various dissociation reactions are in good agreement with the previous theoretical/experimental values. Low-lying vibrational energy levels of ZnH2, ZnD2, and HZnD are calculated on the three-dimensional potential energy surface using the Lanczos algorithm, and found to be in good agreement with the available experimental band origins and the previous theoretical values. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source]

QM/MM calculation of solvent effects on absorption spectra of guanine

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2010
Maja Parac
Abstract Electronic spectra of guanine in the gas phase and in water were studied by quantum mechanical/molecular mechanical (QM/MM) methods. Geometries for the excited-state calculations were extracted from ground-state molecular dynamics (MD) simulations using the self-consistent-charge density functional tight binding (SCC-DFTB) method for the QM region and the TIP3P force field for the water environment. Theoretical absorption spectra were generated from excitation energies and oscillator strengths calculated for 50 to 500 MD snapshots of guanine in the gas phase (QM) and in solution (QM/MM). The excited-state calculations used time-dependent density functional theory (TDDFT) and the DFT-based multireference configuration interaction (DFT/MRCI) method of Grimme and Waletzke, in combination with two basis sets. Our investigation covered keto-N7H and keto-N9H guanine, with particular focus on solvent effects in the low-energy spectrum of the keto-N9H tautomer. When compared with the vertical excitation energies of gas-phase guanine at the optimized DFT (B3LYP/TZVP) geometry, the maxima in the computed solution spectra are shifted by several tenths of an eV. Three effects contribute: the use of SCC-DFTB-based rather than B3LYP-based geometries in the MD snapshots (red shift of ca. 0.1 eV), explicit inclusion of nuclear motion through the MD snapshots (red shift of ca. 0.1 eV), and intrinsic solvent effects (differences in the absorption maxima in the computed gas-phase and solution spectra, typically ca. 0.1,0.3 eV). A detailed analysis of the results indicates that the intrinsic solvent effects arise both from solvent-induced structural changes and from electrostatic solute,solvent interactions, the latter being dominant. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010 [source]

The VMFCI method: A flexible tool for solving the molecular vibration problem

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2006
P. Cassam-Chenaï
Abstract The present article introduces a general variational scheme to find approximate solutions of the spectral problem for the molecular vibration Hamiltonian. It is called the "vibrational mean field configuration interaction" (VMFCI) method, and consists in performing vibrational configuration interactions (VCI) for selected modes in the mean field of the others. The same partition of modes can be iterated until self-consistency, generalizing the vibrational self-consistent field (VSCF) method. As in contracted-mode methods, a hierarchy of partitions can be built to ultimately contract all the modes together. So, the VMFCI method extends the traditional variational approaches and can be included in existing vibrational codes based on the latter approaches. The flexibility and efficiency of this new method are demonstrated on several molecules of atmospheric interest. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 627,640, 2006 [source]

Extensive theoretical studies on the low-lying electronic states of indium monochloride cation, InCl+

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2005
Wenli Zou
Abstract The global potential energy curves for the 14 low-lying doublet and quartet ,-S states of InCl+ are calculated at the scalar relativistic MR-CISD+Q (multireference configuration interaction with single and double excitations, and Davidson's correction) level of theory. Spin-orbit coupling is accounted for via the state interaction approach with the full Breit,Pauli Hamiltonian, which leads to 30 , states. The computed spectroscopic constants of nine bound ,-S states and 17 bound , states are in good agreement with the available experimental data. The transition dipole moments and Franck,Condon factors of selected transitions are also calculated, from which the corresponding radiative lifetimes are derived. © 2004 Wiley Periodicals, Inc. J Comput Chem 26: 106,113, 2005 [source]

A practical valence bond method: A configuration interaction method approach with perturbation theoretic facility

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2004
Lingchun Song
Abstract The previously developed valence bond configuration interaction (VBCI) method (Wu, W.; Song, L.; Cao, Z.; Zhang, Q.; Shaik, S., J. Phys. Chem. A, 2002, 105, 2721) that borrows the general CI philosophy of the MO theory, is further extended in this article, and its methodological features are improved, resulting in three accurate and cost-effective procedures: (a) the effect of quadruplet excitation is incorporated using the Davidson correction, such that the new procedure reduces size consistency problems, with due improvement in the quality of the computational results. (b) A cost-effective procedure, named VBCI(D, S), is introduced. It includes doubly excited structures for active electrons and singly excited structures for inactive pairs. The computational results of VBCI(D, S) match those of VBCISD with much less computational effort than VBCISD. (c) Finally, a second-order perturbation theory is utilized as a means of configuration selection, and lead to considerable reduction of the computational cost, with little or no loss in accuracy. Applications of the new procedures to bond energies and barriers of chemical reactions are presented and discussed. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 472,478, 2004 [source]

Systematic quantum chemical study of DNA-base tautomers

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2004
M. Piacenza
Abstract The relative energies of the energetically low-lying tautomers of pyridone, cytosine, uracil, thymine, guanine, and iso-cytosine are studied by a variety of different quantum chemical methods. In particular, we employ density functional theory (DFT) using the six functionals HCTH407, PBE, BP86, B-LYP, B3-LYP, and BH-LYP, and the ab initio methods Hartree-Fock (HF), standard second-order Møller-Plesset perturbation theory (MP2), an improved version of it (SCS-MP2), and quadratic configuration interaction including single and double excitations (QCISD) and perturbative triple corrections [QCISD(T)]. A detailed basis set study is performed for the formamide/formamidic acid tautomeric pair. In general, large AO basis sets of at least valence triple-, quality including f-functions (TZV) are employed, which are found to be necessary for an accurate energetic description of the various structures. The performance of the more approximate methods is evaluated with QCISD(T)/TZV(2df,2dp) data taken as reference. In general it is found that DFT is not an appropriate method for the problem. For the tautomers of pyridone and cytosine, most density functionals, including the popular B3-LYP hybrid, predict a wrong energetic order, and only for guanine, the correct sequence of tautomers is obtained with all functionals. Out of the density functionals tested, BH-LYP, which includes a rather large fraction of HF exchange, performs best. A consistent description of the nonaromatic versus aromatic tautomers seems to be a general problem especially for pure, nonhybrid functionals. Tentatively, this could be assigned to the exchange potentials used while the functional itself, including the correlation part, seems to be appropriate. Out of the ab initio methods tested, the new SCS-MP2 approach seems to perform best because it effectively reduces some outliers obtained with standard MP2. It outperforms the much more costly QCISD method and seems to be a very good compromise between computational effort and accuracy. © 2003 Wiley Periodicals, Inc. J Comput Chem 1: 83,98, 2004 [source]

Systematic Study of Selected Diagonalization Methods for Configuration Interaction Matrices

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2001
Matthew L. Leininger
Abstract Several modifications to the Davidson algorithm are systematically explored to establish their performance for an assortment of configuration interaction (CI) computations. The combination of a generalized Davidson method, a periodic two-vector subspace collapse, and a blocked Davidson approach for multiple roots is determined to retain the convergence characteristics of the full subspace method. This approach permits the efficient computation of wave functions for large-scale CI matrices by eliminating the need to ever store more than three expansion vectors (bi) and associated matrix-vector products (,i), thereby dramatically reducing the I/O requirements relative to the full subspace scheme. The minimal-storage, single-vector method of Olsen is found to be a reasonable alternative for obtaining energies of well-behaved systems to within ,Eh accuracy, although it typically requires around 50% more iterations and at times is too inefficient to yield high accuracy (ca. 10,10Eh) for very large CI problems. Several approximations to the diagonal elements of the CI Hamiltonian matrix are found to allow simple on-the-fly computation of the preconditioning matrix, to maintain the spin symmetry of the determinant-based wave function, and to preserve the convergence characteristics of the diagonalization procedure. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1574,1589, 2001 [source]

Characterization of Fe 3d states in CuFeS2 by resonant X-ray emission spectroscopy

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2009
Katsuaki Sato
Abstract Resonant X-ray emission spectroscopy (RXES) experiments were carried out in a single crystal of chalcopyrite CuFeS2, an antiferromagnetic semiconductor with a golden lustre, to unravel the overlapping d,d and charge-transfer transitions extending well above the absorption edge, which cannot be observed by conventional optical absorption experiments. The observed RXES spectra have been analyzed by means of cluster-model calculation with configuration interaction, which leads to the conclusion that CuFeS2 is a Haldane,Anderson insulator with a negative value of charge transfer energy, , = ,3 eV. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Electronic excitations and optical spectra of Pt2 and Pt4 on Cu(001) modeled by a cluster

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2010
George Pal
Abstract The photoabsorption spectra of Pt2 and Pt4 clusters on the Cu(001) surface are computed using two different theoretical methods: the symmetry-adapted cluster expansion configuration interaction from quantum chemistry and a recently developed linear response approach to treat electron,hole correlations in the presence of an external electromagnetic field. Comparing the energetically low-lying optical excitations, we find very good agreement between the two methods. For different orientations of the adsorbed clusters with respect to the surface, we find that the most intense optical peaks occur when the polarization of the applied laser pulse is parallel to the surface. [source]

The Excited-State Chemistry of Phycocyanobilin: A Semiempirical Study

CHEMPHYSCHEM, Issue 7 2005
Andreas H. Göller Dr.
Abstract Based on previous time-resolved absorption studies, phycocyanobilin undergoes a photoreaction from an A - into a B - and C -form, with the latter two photoproducts showing absorption spectra red-shifted from A. To identify the molecular mechanism involved in the excited-state reactions, the structural origin of the red shift in the absorption spectra is investigated. Using semiempirical AM1 calculations that include configuration interaction by pair doubles excitation configuration interaction, the absorption spectra of different conformers as well as different protonation states were calculated. The results clearly indicate a pronounced red shift in the spectra of structures either protonated or deprotonated at the basic/acidic centres of the tetrapyrrole chromophore whereas, in contrast, conformational changes alone result in a blue shift. Furthermore, it is shown by quantum chemical calculations that the basicity of phycocyanobilin is much higher in the excited than in the ground state, with a decrease in the excited-state pKB* of ,9.5 units. The acidity is only slightly enhanced with a drop in pKA* of only ,1.6 units. From these findings, a reaction model for the excited-state processes in phycocyanobilin is proposed. According to this model, photoexcitation of phycocyanobilin triggers an excited-state proton transfer giving rise to the formation of a protonated species. In parallel, the local increase in the medium pH associated with protonation then forwards a deprotonation at an acidic NH-group so that in effect both protonated and deprotonated phycocyanobilin would arise from the initial photoreaction and account for the observed red shift in the spectra of the B - and C -forms. [source]