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Correlation Effects (correlation + effects)
Kinds of Correlation Effects Selected AbstractsCorrelation effects in low dimensional electron systems: the electron-hole bilayerCONTRIBUTIONS TO PLASMA PHYSICS, Issue 5-6 2003G. Senatore Abstract We review recent progress in the area of low-dimensional electrons, which has been made possible mainly by the use of quantum simulations. We focus on DMC results for a symmetric electron-hole bilayer, which we analyze in some detail. We also contrast the results of simulations with (i) the predictions of a mean field treatments and (ii) the results of approaches based on the dielectric formalism, such as the STLS and qSTLS schemes. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Volatility linkages of the equity, bond and money markets: an implied volatility approachACCOUNTING & FINANCE, Issue 1 2009Kent Wang G12; G14 Abstract This study proposes an alternative approach for examining volatility linkages between Standard & Poor's 500, Eurodollar futures and 30 year Treasury Bond futures markets using implied volatility from the three markets. Simple correlation analysis between implied volatilities in the three markets is used to assess market correlations. Spurious correlation effects are considered and controlled for. I find that correlations between implied volatilities in the equity, money and bond markets are positive, strong and robust. Furthermore, I replicate the approach of Fleming, Kirby and Ostdiek (1998) to check the substitutability of the implied volatility approach and find that the results are nearly identical; I conclude that my approach is simple, robust and preferable in practice. I also argue that the results from this paper provide supportive evidence on the information content of implied volatilities in the equity, bond and money markets. [source] The electronic and optical properties of oligo(trans -1,2-di(2-thienyl)-1,3-butadiene): A theoretical studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2008Nei Marçal Abstract In the present work we investigated the theoretical electronic structure of poly(trans -1,4-di(2-thienyl)-1,3-butadiene) (PTB) and determined the optical properties of its neutral and doped oligomers. Geometrical optimizations were at the semiempirical level by using the Austin method 1 (AM1). The band structure of , electrons regarding to the neutral PTB polymer was obtained by using a tight-binding Hamiltonian. The densities of electronic states (DOS) for neutral and doped copolymers were calculated by using the negative factor counting technique. The spatial charge distribution of the oligomeric chain was also analyzed. The energy of the electronic transitions and their associated oscillator strength values were calculated for the neutral, double, and single charged oligomers to determine the UV,vis absorption spectra. The calculations were performed using the intermediate neglect of differential overlap Hamiltonian in combination with the single configuration-interaction technique in order to include correlation effects. The band gap obtained in the PTB was about 2.101 eV for the optics absorption and 1.73 eV for the DOS. The bipolaron states appear in the gap, about 0.57 eV and 0.48 eV below and above the conduction and valence bands, respectively. When the dopants concentration is increased the DOS showed that the energy gap tends to vanish, which may lead to semiconductor,metal transition. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Comparison of second-order orbital-dependent DFT correlation functionalsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 12 2008Ireneusz Grabowski Abstract The choice of the orbital-dependent second-order correlation functional plays the prime role in the description of the correlation effects in orbital-dependent DFT calculations. Using second-order perturbation theory we were able to derive the simplest orbital-dependent correlation functional, but even at this lowest correlation level, we had several possibilities to define it. Applications of different second-order correlation functionals for the atomic as well as molecular systems are presented. The ab initio DFT-type OEP2 functionals based on Møller-Plesset or semicanonical partitioning (OEP2-sc) are compared with those based on Epstein-Nesbet type partitioning, showing that the latter ones can fail in more difficult molecular problems, e.g., the Be dimer potential curve. We show that currently the best performing orbital-dependent second-order correlation functional is the OEP2-sc one. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Spin-optimized resonating Hartree-Fock configuration interactionINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2007Ryo 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] Non,Born,Oppenheimer calculations of the ground state of H3INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2007Mauricio Cafiero Abstract We present quantum,mechanical calculations for the ground state of the H3 system performed without the Born-Oppenehimer approximation. In the calculations we use explicitly correlated Gaussian basis functions that explicitly depend on all of the interparticle distances. These basis functions allow us to achieve high accuracy while explicitly describing nucleus,nucleus, nucleus,electron, and electron,electron correlation effects. Gaussian basis sets ranging in size from 85 to 950 functions have been optimized using a gradient-based procedure. The issue of defining and extracting the H3 molecular structure based on the non-BO wave function is also discussed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Ab initio investigation of the solvent and electron correlation effects on the geometries and first hyperpolarizabilities of push,pull oligomersINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 11 2007Eric A. Perpète Abstract Using the HF and MP2 approaches, we have determined the ground-state geometry and first hyperpolarizability of increasingly long push,pull polyacetylene oligomers. The bulk solvent effects have been assessed at both levels of theory through the Polarizable Continuum Model. For the first hyperpolarizability, the resulting 16 combination of theoretical levels of computation have been compared to evaluate the importance of individual corrections, as well as their additive/ multiplicative character. It turns out that the inclusion of bulk solvent effects is essential for an accurate estimate of the nonlinear optics properties of these push,pull derivatives. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Soft Coulomb hole method applied to moleculesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2007J. Ortega-Castro Abstract The soft Coulomb hole method introduces a perturbation operator, defined by ,e/r12 to take into account electron correlation effects, where , represents the width of the Coulomb hole. A new parametrization for the soft Coulomb hole operator is presented with the purpose of obtaining better molecular geometries than those resulting from Hartree,Fock calculations, as well as correlation energies. The 12 parameters included in , were determined for a reference set of 12 molecules and applied to a large set of molecules (38 homo- and heteronuclear diatomic molecules, and 37 small and medium-size molecules). For these systems, the optimized geometries were compared with experimental values; correlation energies were compared with results of the MP2, B3LYP, and Gaussian 3 approach. On average, molecular geometries are better than the Hartree,Fock values, and correlation energies yield results halfway between MP2 and B3LYP. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Thomas,Fermi approximation for the quasi-two-dimensional electron gasINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2001M. Moreno Abstract To take into account static correlation effects in the quasi-two-dimensional electron gas a screened Coulombic interaction between particles is studied. The Thomas,Fermi approximation is used and the potential screening appears as a function of the Wigner,Seitz density parameter rs and the effective width t of the system. With the self-consistent field theory applied to the modified deformable jellium, the ground-state energy per particle and the conditions for electron localization are obtained in terms of the interparticle distance and the screening parameter ,. A critical minimum characteristic width tc is obtained; below tc no long-range order is obtained. For larger widths a stable localized state is predicted at finite densities. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 269,276, 2001 [source] Ab-initio simulations of materials using VASP: Density-functional theory and beyondJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2008Jürgen Hafner Abstract During the past decade, computer simulations based on a quantum-mechanical description of the interactions between electrons and between electrons and atomic nuclei have developed an increasingly important impact on solid-state physics and chemistry and on materials science,promoting not only a deeper understanding, but also the possibility to contribute significantly to materials design for future technologies. This development is based on two important columns: (i) The improved description of electronic many-body effects within density-functional theory (DFT) and the upcoming post-DFT methods. (ii) The implementation of the new functionals and many-body techniques within highly efficient, stable, and versatile computer codes, which allow to exploit the potential of modern computer architectures. In this review, I discuss the implementation of various DFT functionals [local-density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, hybrid functional mixing DFT, and exact (Hartree-Fock) exchange] and post-DFT approaches [DFT + U for strong electronic correlations in narrow bands, many-body perturbation theory (GW) for quasiparticle spectra, dynamical correlation effects via the adiabatic-connection fluctuation-dissipation theorem (AC-FDT)] in the Vienna ab initio simulation package VASP. VASP is a plane-wave all-electron code using the projector-augmented wave method to describe the electron-core interaction. The code uses fast iterative techniques for the diagonalization of the DFT Hamiltonian and allows to perform total-energy calculations and structural optimizations for systems with thousands of atoms and ab initio molecular dynamics simulations for ensembles with a few hundred atoms extending over several tens of ps. Applications in many different areas (structure and phase stability, mechanical and dynamical properties, liquids, glasses and quasicrystals, magnetism and magnetic nanostructures, semiconductors and insulators, surfaces, interfaces and thin films, chemical reactions, and catalysis) are reviewed. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] An improved algorithm for analytical gradient evaluation in resolution-of-the-identity second-order Møller-Plesset perturbation theory: Application to alanine tetrapeptide conformational analysisJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2007Robert A. Distasio JR. Abstract We present a new algorithm for analytical gradient evaluation in resolution-of-the-identity second-order Møller-Plesset perturbation theory (RI-MP2) and thoroughly assess its computational performance and chemical accuracy. This algorithm addresses the potential I/O bottlenecks associated with disk-based storage and access of the RI-MP2 t -amplitudes by utilizing a semi-direct batching approach and yields computational speed-ups of approximately 2,3 over the best conventional MP2 analytical gradient algorithms. In addition, we attempt to provide a straightforward guide to performing reliable and cost-efficient geometry optimizations at the RI-MP2 level of theory. By computing relative atomization energies for the G3/99 set and optimizing a test set of 136 equilibrium molecular structures, we demonstrate that satisfactory relative accuracy and significant computational savings can be obtained using Pople-style atomic orbital basis sets with the existing auxiliary basis expansions for RI-MP2 computations. We also show that RI-MP2 geometry optimizations reproduce molecular equilibrium structures with no significant deviations (>0.1 pm) from the predictions of conventional MP2 theory. As a chemical application, we computed the extended-globular conformational energy gap in alanine tetrapeptide at the extrapolated RI-MP2/cc-pV(TQ)Z level as 2.884, 4.414, and 4.994 kcal/mol for structures optimized using the HF, DFT (B3LYP), and RI-MP2 methodologies and the cc-pVTZ basis set, respectively. These marked energetic discrepancies originate from differential intramolecular hydrogen bonding present in the globular conformation optimized at these levels of theory and clearly demonstrate the importance of long-range correlation effects in polypeptide conformational analysis. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] A theoretical investigation of the excited states of OCLO radical, cation, and anion using the CASSCF/CASPT2 methodJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2007Zi-Zhang Wei Abstract Using the complete active space self-consistent field method with a large atomic natural orbital basis set, 10, 13, and 9 electronic states of the OClO radical, OClO+ cation, and OClO, anion were calculated, respectively. Taking the further correlation effects into account, the second-order perturbation (CASPT2) calculations were carried out for the energetic calibration. The photoelectron spectroscopy of the OClO radical and OClO, anion were extensively studied in the both case of the adiabatic and vertical ionization energies. The calculated results presented the relatively complete assignment of the photoelectron bands of the experiments for OClO and its anion. Furthermore, the Rydberg states of the OClO radical were investigated by using multiconfigurational CASPT2 (MS-CASPT2) theory under the basis set of large atomic natural orbital functions augmented with an adapted 1s1p1d Rydberg functions that have specially been built for this study. Sixteen Rydberg states were obtained and the results were consistent with the experimental results. © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 467,477, 2007 [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] Oxygen Diffusion in Yttria-Stabilized Zirconia: A New Simulation ModelJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2004R. Krishnamurthy We present a multiscale modeling approach to study oxygen diffusion in cubic yttria-stabilized zirconia. In this approach, we employ density functional theory methods to calculate activation energies for oxygen migration in different cation environments. These are used in a kinetic Monte Carlo framework to calculate long-time oxygen diffusivities. Simulation results show that the oxygen diffusivity attains a maximum value at around 0.1 mole fraction yttria. This variation in the oxygen diffusivity with yttria mole fraction and the calculated values for the diffusivity agree well with experiment. The competing effects of increased oxygen vacancy concentration and increasing activation energy and correlation effects for oxygen diffusion with increasing yttria mole fraction are responsible for the observed dopant content dependence of the oxygen diffusivity. We provide a detailed analysis of cation-dopant-induced correlation effects in support of the above explanation. [source] Influence of the correlation effects on charge transport through quantum dotsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2007T. Doma Abstract We study the effect of the Coulomb interactions on a charge transport through a single level quantum dot. The excitation spectrum is at low temperatures characterized by a narrow Kondo resonance (the low energy feature) formed on top of the line broadening (high energy part of the spectrum). We investigate mutual interplay between these low and high energy parts and discuss their influence on the transport phenomena for a semi-equilibrium situation. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nonadiabatic electron,phonon effects in low carrier density superconductorsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2005E. Cappelluti Abstract Different families of unconventional superconductors present a low charge carrier density as a common trait, suggesting that the low charge density can be at the basis of a unifying picture for different superconductors. In the past years we have suggested that the electron,phonon interaction can be responsible for a high- Tc superconducting pairing in a nonadiabatic regime, where nonadiabatic effects are triggered on by the small electronic Fermi energy associated with the low charge density character. A coherent picture of such a framework requires however reconciling the low charge density and the small Fermi energy with a finite metallic character (sizable density of states and large Fermi surfaces). In this paper we investigate the peculiar conditions which are needed to be encountered in order to fulfill these requirements. We discuss the specific case of fullerenes, cuprates and MgB2 alloys by analyzing their specific structural and electronic properties The comparison between these materials and simple instructive models permits to underline the different routes to reconcile these characteristics in different compounds. In cuprates and fullerenes the interplay between small Fermi energies and large Fermi surface is strictly connected with strong electronic correlation effects. A comprehensive understanding of these issues can be useful to the future search for new nonadiabatic high- Tc materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Quasi-localized low-frequency vibrational modes of disordered solids: Study by single-molecule spectroscopyPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2004A. V. Naumov Editor's Choice of this issue of physica status solidi (b) is the article [1] by Andrei V. Naumov et al. This paper is Part II (Part I see [2]) of a study on elementary excitations in glasses, presented at the 11th International Conference on Phonon Scattering in Condensed Matter, St. Petersburg, 25,30 July 2004. For his outstanding talk, Naumov received the new physica status solidi Young Researcher Award which was bestowed for the first time at this conference. The cover picture is a sketch of a glass with a single impurity molecule and one hypothetical quasi-localized vibrational mode. The broadening and shift of the chromophore spectral line are caused by the interaction with this mode. Andrei V. Naumov is senior scientific researcher and deputy head of the Molecular Spectroscopy Department of the Institute of Spectroscopy, Troitsk. His main research interests are experimental and theoretical studies of low-temperature dynamics of amorphous solids (glasses, polymers etc.) via high resolution laser selective spectroscopy techniques. The second Editor's Choice is an article by E. A. Eliseev and M. D. Glinchuk [3]. Eugene A. Eliseev is scientific researcher at the Frantsevich Institute for Problems of Materials Science of the Ukrainian National Academy of Sciences, Kiev. His research areas are the theory of size and correlation effects in ferroelectric materials as well as modelling of disordered ferroelectrics properties. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Credit risk and bank margins in structured financial products: Evidence from the German secondary market for discount certificatesTHE JOURNAL OF FUTURES MARKETS, Issue 4 2008Rainer Baule This study analyzes bank margins in the German secondary market for exchange-traded structured financial products, with particular emphasis on the influence of banks' credit risk. A structural model allowing for the incorporation of correlation effects between market and credit risk is applied to compare quoted and fair theoretical prices. For discount certificates, as the most popular type of structured financial products in Germany, an empirical study is conducted. Compared to earlier studies, total margins are found to be rather low, whereas the portion that draws back to credit risk appears to be a material part of the total margin. © 2008 Wiley Periodicals, Inc. Jrl Fut Mark 28: 376,397, 2008 [source] On the origin of topological differences between experimental and theoretical crystal charge densitiesACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2000Anatoliy Volkov Topological analysis of experimental and theoretical (molecular and crystal) electron densities of p -nitroaniline and p -amino- p,-nitrobiphenyl reveals considerable discrepancies between experiment and theory for the bond critical points properties. Particularly large differences occur for the positive curvature along the bond path (,3). The differences become somewhat smaller when more extended basis sets and correlation effects are introduced in the theoretical calculations. The effect of the crystal matrix on the properties of bond critical points is evaluated for the p -nitroaniline molecule using the 6-21G** and 6-31G** basis sets. The differences between the isolated molecule and the molecule in the crystal are too small to explain the quantitative disagreement between the theoretical and experimental topologies reported in the literature and found in the current study. For most bonds, the observed changes in the properties of the electron density agree well for both basis sets but some discrepancies are found for changes in ,3 for N,H and aromatic C,C bonds. When the theoretical densities are projected into the multipole density functions through refinement of the theoretical structure factors, the topological properties change and differences between theory and experiment are reduced. The main origin of the observed discrepancies is attributed to the nature of the radial functions in the experimental multipole model. [source] Electronic structure and transport properties of quantum dotsANNALEN DER PHYSIK, Issue 5 2004M. Tews Abstract The subject of this paper are electronic properties of isolated quantum dots as well as transport properties of quantum dots coupled to two electronic reservoirs. Thereby special focus is put on the effects of Coulomb interaction and possible correlations in the quantum dot states. First, the regime of sequential tunneling to the reservoirs is investigated. It is shown that in case degenerate states participate in transport, the resonance positions in the differential conductance generally depend on temperature and the degree of degeneracy. This effect can be used to directly probe degeneracies in a quantum dot spectrum. A further effect, characteristic for sequential tunneling events, is the complete blocking of individual channels for transport. A generalisation of the well known spin blockade is found for correlated dot states transitions through which are not directly spin-forbidden. In the second part, the electronic structure of spherical quantum dots is calculated. In order to account for correlation effects, the few-particle Schrödinger equation is solved by an exact diagonalization procedure. The calculated electronic structure compares to experimental findings obtained on colloidal semiconductor nanocrystals by Scanning Tunneling Spectroscopy. It is found that the electric field induced by the tunneling tip is gives rise to a Stark effect which can break the spherical symmetry of the electronic ground state density which is in agreement with wave-function mapping experiments. The symmetry breaking depends on the competition between exchange energy and the Stark energy. Moreover, a systematic dependence on particle number is found for the excitation energies of optical transitions which explains recent experimental findings on self-organized quantum dots. In the last part, co-tunneling in the Coulomb blockade regime is studied. For this end the tunneling current is calculated up to the forth order perturbation theory in the tunnel coupling by a real-time Green's function approach for the non-equilibrium case. The differential conductance calculated for a quantum dot containing up to two interacting electrons shows complex signatures of the excitation spectrum which are explained by a combination of co-tunneling and sequential tunneling processes. Thereby the calculations show a peak structure within the Coulomb blockade regime which has also been observed in experiment. [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] Ab initio Study of the Interactions between CO2 and N-Containing Organic HeterocyclesCHEMPHYSCHEM, Issue 2 2009Konstantinos D. Vogiatzis Abstract In the garden of dispersion: High-accuracy ab initio calculations are performed to determine the nature of the interactions and the most favorable geometries between CO2 and heteroaromatic molecules containing nitrogen (see figure). Dispersion forces play a key role in the stabilization of the dimer, because correlation effects represent about 50,% of the total interaction energy. The interactions between carbon dioxide and organic heterocyclic molecules containing nitrogen are studied by using high-accuracy ab initio methods. Various adsorption positions are examined for pyridine. The preferred configuration is an in-plane configuration. An electron donor,electron acceptor (EDA) mechanism between the carbon of CO2 and the nitrogen of the heterocycle and weak hydrogen bonds stabilize the complex, with important contributions from dispersion and induction forces. Quantitative results of the binding energy of CO2 to pyridine (C5H5N), pyrimidine, pyridazine, and pyrazine (C4H4N2), triazine (C3H3N3), imidazole (C3H4N2), tetrazole (CH2N4), purine (C5H4N4), imidazopyridine (C6H5N3), adenine (C5H5N5), and imidazopyridamine (C6H6N4) for the in-plane configuration are presented. For purine, three different binding sites are examined. An approximate coupled-cluster model including single and double excitations with a perturbative estimation of triple excitations (CCSD(T)) is used for benchmark calculations. The CCSD(T) basis-set limit is approximated from explicitly correlated second-order Møller,Plesset (MP2-F12) calculations in the aug-cc-pVTZ basis in conjunction with contributions from single, double, and triple excitations calculated at the CCSD(T)/6-311++G** level of theory. Extrapolations to the MP2 basis-set limit coincide with the MP2-F12 calculations. The results are interpreted in terms of electrostatic potential maps and electron density redistribution plots. The effectiveness of density functional theory with the empirical dispersion correction of Grimme (DFT-D) is also examined. [source] | ||||||||||||||||