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Quantum Chemistry (quantum + chemistry)
Terms modified by Quantum Chemistry Selected AbstractsHow many-body perturbation theory (MBPT) has changed quantum chemistryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2009Werner KutzelniggArticle first published online: 26 AUG 200 Abstract The history of many-body perturbation theory (MBPT) and its impact on Quantum Chemistry is reviewed, starting with Brueckner's conjecture of a linked-cluster expansion and the time-dependent derivation by Goldstone of such an expansion. A central part of this article is the time-independent formulation of quantum chemistry in Fock space and its diagrammatic representation including the particle-hole picture and the inversion of a commutator. The results of the time-independent derivation of MBPT are compared with those of Goldstone. It is analyzed which ingredients of Goldstone's approach are decisive. The connected diagram theorem is derived both in a constructive way based on a Lie-algebraic formulation and a nonconstructive way making use of the separation theorem. It is discussed why the Goldstone derivation starting from a unitary time-evolution operator, ends up with a wave operator in intermediate normalization. The Møller,Plesset perturbation expansions of Bartlett and Pople are compared. Examples of complete summations of certain classes of diagrams are discussed, for example, that which leads to the Bethe-Goldstone expansion. MBPT for energy differences is analyzed. The paper ends with recent developments and challenges, such as the generalization of normal ordering to arbitrary reference states, contracted Schrödinger k -particle equations and Brillouin conditions, and finally the Nakatsuji theorem and the Nooijen conjecture. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Lulu Huang, Lou Massa, Jerome Karle, "Kernel energy method illustrated with peptides," Internationl Journal of Quantum Chemistry (2005) 103(6) 808,817INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2006Article first published online: 14 NOV 200 No abstract is available for this article. [source] Erratum: Spectroscopic constants of H2 using Monte Carlo methods, S.A. Alexander and R.L. Coldwell, International Journal of Quantum Chemistry(2004) 100(6) 851,857INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2005S. A. Alexander The original article to which this Erratum refers was published in International Journal of Quantum Chemistry (2004) 100(6)851,857 [source] Book Review: Handbook of Molecular Physics and Quantum Chemistry.CHEMPHYSCHEM, Issue 5 2004Editors Stephen Wilson, Peter F. Bernath, Roy McWeeny No abstract is available for this article. [source] Metric spaces in NMR crystallographyCONCEPTS IN MAGNETIC RESONANCE, Issue 4 2009David M. Grant Abstract The anisotropic character of the chemical shift can be measured by experiments that provide shift tensor values and comparing these experimental components, obtained from microcrystalline powders, with 3D nuclear shielding tensor components, calculated with quantum chemistry, yields structural models of the analyzed molecules. The use of a metric tensor for evaluating the mean squared deviations, d2, between two or more tensors provides a statistical way to verify the molecular structure governing the theoretical shielding components. The sensitivity of the method is comparable with diffraction methods for the heavier organic atoms (i.e., C, O, N, etc.) but considerably better for the positions of H atoms. Thus, the method is especially powerful for H-bond structure, the position of water molecules in biomolecular species, and other proton important structural features, etc. Unfortunately, the traditional Cartesian tensor components appear as reducible metric representations and lack the orthogonality of irreducible icosahedral and irreducible spherical tensors, both of which are also easy to normalize. Metrics give weighting factors that carry important statistical significance in a structure determination. Details of the mathematical analysis are presented and examples given to illustrate the reason nuclear magnetic resonance are rapidly assuming an important synergistic relationship with diffraction methods (X-ray, neutron scattering, and high energy synchrotron irradiation). © 2009 Wiley Periodicals, Inc.Concepts Magn Reson Part A 34A: 217,237, 2009. [source] Myth and Reality in the Attitude toward Valence-Bond (VB) Theory: Are Its ,Failures' Real?HELVETICA CHIMICA ACTA, Issue 4 2003Sason Shaik According to common wisdom propagated in textbooks and papers, valence-bond (VB) theory fails and makes predictions in contradiction with experiment. Four iconic ,failures' are: a) the wrong prediction of the ground state of the O2 molecule, b) the failure to predict the properties of cyclobutadiene (CBD) viz. those of benzene, c) the failure to predict the aromaticity/anti-aromaticity of molecular ions like C5H and C5H, C3H and C3H, C7H and C7H, etc; and d) the failure to predict that, e.g., CH4 has two different ionization potentials. This paper analyzes the origins of these ,failures' and shows that two of them (stated in a and d) are myths of unclear origins, while the other two originate in misuse of an oversimplified version of VB theory, i.e., simple resonance theory that merely enumerate resonance structures. It is demonstrated that, in each case, a properly used VB theory at a simple and portable level leads to correct predictions, as successful as those made by use of molecular-orbital (MO) theory. This notion of VB ,failure', which is traced back to the VB-MO rivalry, in the early days of quantum chemistry, should now be considered obsolete, unwarranted, and counterproductive. A modern chemist should know that there are two ways of describing electronic structure, which are not two contrasting theories, but rather two representations or two guises of the same reality. Their capabilities and insights into chemical problems are complementary, and the exclusion of any one of them undermines the intellectual heritage of chemistry. [source] Erratum: Inverse problems in quantum chemistryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2010Jacek Karwowski No abstract is available for this article. [source] How many-body perturbation theory (MBPT) has changed quantum chemistryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2009Werner KutzelniggArticle first published online: 26 AUG 200 Abstract The history of many-body perturbation theory (MBPT) and its impact on Quantum Chemistry is reviewed, starting with Brueckner's conjecture of a linked-cluster expansion and the time-dependent derivation by Goldstone of such an expansion. A central part of this article is the time-independent formulation of quantum chemistry in Fock space and its diagrammatic representation including the particle-hole picture and the inversion of a commutator. The results of the time-independent derivation of MBPT are compared with those of Goldstone. It is analyzed which ingredients of Goldstone's approach are decisive. The connected diagram theorem is derived both in a constructive way based on a Lie-algebraic formulation and a nonconstructive way making use of the separation theorem. It is discussed why the Goldstone derivation starting from a unitary time-evolution operator, ends up with a wave operator in intermediate normalization. The Møller,Plesset perturbation expansions of Bartlett and Pople are compared. Examples of complete summations of certain classes of diagrams are discussed, for example, that which leads to the Bethe-Goldstone expansion. MBPT for energy differences is analyzed. The paper ends with recent developments and challenges, such as the generalization of normal ordering to arbitrary reference states, contracted Schrödinger k -particle equations and Brillouin conditions, and finally the Nakatsuji theorem and the Nooijen conjecture. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Inverse problems in quantum chemistryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 11 2009Jacek Karwowski Abstract Inverse problems constitute a branch of applied mathematics with well-developed methodology and formalism. A broad family of tasks met in theoretical physics, in civil and mechanical engineering, as well as in various branches of medical and biological sciences has been formulated as specific implementations of the general theory of inverse problems. In this article, it is pointed out that a number of approaches met in quantum chemistry can (and should) be classified as inverse problems. Consequently, the methodology used in these approaches may be enriched by applying ideas and theorems developed within the general field of inverse problems. Several examples, including the RKR method for the construction of potential energy curves, determining parameter values in semiempirical methods, and finding external potentials for which the pertinent Schrödinger equation is exactly solvable, are discussed in detail. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Central moments in quantum chemistryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 7 2008David W. Small Abstract We define central moments of operators on finite-dimensional vector spaces and study some of their basic aspects. Central moments may be viewed as generalizations of the dispersion of a Hermitian operator. We show how eigenvalues may be represented by central moments, and how central moments may be used to obtain Krylov subspace approximations for operators on inner product spaces. We show that central-moments approximations are compatible with the concepts of size-consistency in quantum chemistry, and we use this to suggest a foundation for central-moments approximations in Coupled Cluster theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Group functions approach based on the combination of strictly local geminals and molecular orbitalsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2006A. M. Tokmachev Abstract Thegroup functions technique is a natural way to introduce local description into quantum chemistry. It can also be a basis for construction of numerically effective computational schemes having almost linear growth of computational costs with that of the size of the system. Previously, we constructed a family of computationally efficient semiempirical methods based on the variationally determined strictly local geminals (SLGs). It was implemented with four popular parameterization schemes (MINDO/3, MNDO, AM1, and PM3). Because of construction details, its applicability was restricted only to compounds with well-defined two-electron two-center chemical bonds and lone pairs. We generalize the previous treatment to make the electronic structure calculations possible for a wider class of compounds without loss of computational efficiency. The proposed scheme (SLG/SCF) is based on the general group function approach combining different descriptions for different electron groups: essentially local two-electron ones are described by geminals, while those with other numbers of electrons are described in the one-electron approximation. We implement the RHF, UHF, and ROHF approaches for the groups with delocalized electrons. This approach is tested for a series of radicals and molecules with extended ,-electron systems. It is shown that the SLG/SCF-based methods describe the experimental data not worse than the corresponding SCF procedures and provide a good starting point for calculations of polyatomic molecular systems. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] From quantum chemistry and the classical theory of polar liquids to continuum approximations in molecular mechanics calculations,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005Sergio A. Hassan Abstract Biological macromolecules and other polymers belong to the class of mesoscopic systems, with characteristic length scale of the order of a nanometer. Although microscopic models would be the preferred choice in theoretical calculations, their use in computer simulations becomes prohibitive for large systems or long simulation times. On the other hand, the use of purely macroscopic models in the mesoscopic domain may introduce artifacts, with effects that are difficult to assess and that may compromise the reliability of the calculations. Here is proposed an approach with the aim of minimizing the empirical nature of continuum approximations of solvent effects within the scope of molecular mechanics (MM) approximations in mesoscopic systems. Using quantum chemical methods, the potential generated by the molecular electron density is first decomposed in a multicenter-multipole expansion around predetermined centers. The monopole and dipole terms of the expansion at each site create electric fields that polarize the surrounding aqueous medium whose dielectric properties can be described by the classical theory of polar liquids. Debye's theory allows a derivation of the dielectric profiles created around isolated point charges and dipoles that can incorporate Onsager reaction field corrections. A superposition of screened Coulomb potentials obtained from this theory makes possible a simple derivation of a formal expression for the total electrostatic energy and the polar component of the solvation energy of the system. A discussion is presented on the physical meaning of the model parameters, their transferability, and their convergence to calculable quantities in the limit of simple systems. The performance of this continuum approximation in computer calculations of amino acids in the context of an atomistic force field is discussed. Applications of a continuum model based on screened Coulomb potentials in multinanosecond simulations of peptides and proteins are briefly reviewed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Theoretical study of the microhydration of mononuclear and dinuclear uranium(VI) species derived from solvolysis of uranyl nitrate in waterJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2010Milan On Abstract The structures and energetics of mononuclear and dinuclear uranium species formed upon speciation of uranyl(VI) nitrate, UO2(NO3)2, in water are investigated by quantum chemistry using density functional theory and the wavefunction-based methods (MP2, CCSD, CCSD(T)). We provide a discussion of the basic coordination patterns of the various mono- and dinuclear uranyl compounds [(UO2)m(X,Y)2m,1(H2O)n]+ (m = 1, 2; n = 0,4) found in a recent mass spectrometric study (Tsierkezos et al., Inorg Chem 2009, 48, 6287). The energetics of the complexation of the uranyl dication to the counterions OH, and NO3, as well as the degradation of the dinuclear species were studied by reference to a test set of 16 representative molecules with the MP2 method and the B3LYP, M06, M06-HF, and M06-2X DFT functionals. All DFT functionals provide structures and energetics close to MP2 results, with M06 family being slightly superior to the standard B3LYP functional. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010 [source] Interactions of the "piano-stool" [ruthenium(II) (,6 -arene)(en)CL]+ complexes with water and nucleobases; ab initio and DFT studyJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2009k Futera Abstract Piano stool ruthenium complexes of the composition [Ru(II)(,6 -arene)(en)Cl]+/2+ (en = ethylenediamine) represent an emerging class of cisplatin-analogue anticancer drug candidates. In this study, we use computational quantum chemistry to characterize the structure, stability and reactivity of these compounds. All these structures were optimized at DFT(B3LYP)/6-31G(d) level and their single point properties were determined by the MP2/6-31++G(2df,2pd) method. Thermodynamic parameters and rate constants were determined for the aquation process, as a replacement of the initial chloro ligand by water and subsequent exchange reaction of aqua ligand by nucleobases. The computations were carried out at several levels of DFT and ab initio theories (B3LYP, MP2 and CCSD) utilizing a range of bases sets (from 6-31G(d) to aug-cc-pVQZ). Excellent agreement with experimental results for aquation process was obtained at the CCSD level and reasonable match was achieved also with the B3LYP/6-31++G(2df,2pd) method. This level was used also for nucleobase-water exchange reaction where a smaller rate constant for guanine exchange was found in comparison with adenine. Although adenine follows a simple replacement mechanism, guanine complex passes by a two-step mechanism. At first, Ru-O6(G) adduct is formed, which is transformed through a chelate TS2 to the Ru-N7(G) final complex. In case of guanine, the exchange reaction is more favorable thermodynamically (releasing in total by about 8 kcal/mol) but according to our results, the rate constant for guanine substitution is slightly smaller than the analogous constant in adenine case when reaction course from local minimum is considered. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Development of a general quantum-chemical descriptor for steric effects: Density functional theory based QSAR study of herbicidal sulfonylurea analoguesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2006Zhen Xi Abstract Quantitative structure-activity relationship (QSAR) analysis has become one of the most effective approaches for optimizing lead compounds and designing new drugs. Although large number of quantum-chemical descriptors were defined and applied successfully, it is still a big challenge to develop a general quantum-chemical descriptor describing the bulk effects more directly and effectively. In this article, we defined a general quantum-chemical descriptor by characterizing the volume of electron cloud for specific substituent using the method of density functional theory. The application of our defined steric descriptors in the QSAR analysis of sulfonylurea analogues resulted in four QSAR models with high quality (the best model: q2 = 0.881, r2 = 0.901, n = 35, s = 0.401, F = 68.44), which indicated that this descriptor may provide an effective way for solving the problem how to directly describe steric effect in quantum chemistry-based QSAR studies. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 1571,1576, 2006 [source] Students' levels of explanations, models, and misconceptions in basic quantum chemistry: A phenomenographic studyJOURNAL OF RESEARCH IN SCIENCE TEACHING, Issue 5 2009Christina Stefani We investigated students' knowledge constructions of basic quantum chemistry concepts, namely atomic orbitals, the Schrödinger equation, molecular orbitals, hybridization, and chemical bonding. Ausubel's theory of meaningful learning provided the theoretical framework and phenomenography the method of analysis. The semi-structured interview with 19 second-year chemistry students supplied the data. We identified four levels of explanations in the students' answers. In addition, the scientific knowledge claims reflected three main levels of models. By combining levels of explanations with levels of models, we derived four categories. Two of the categories are shades of variation in the rote-learning part of a continuum, while the other two categories are in the meaningful-learning part. All students possessed alternative conceptions some of which occurred within certain categories, while others spanned more categories. The insistence on the deterministic models of the atom, the misinterpretation of models, and the poor understanding of the current quantum concepts are main problems in the learning of the basic quantum chemistry concepts. © 2009 Wiley Periodicals, Inc. J Res Sci Teach 46: 520,536, 2009 [source] Electronic excitations and optical spectra of Pt2 and Pt4 on Cu(001) modeled by a clusterPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2010George 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] Hydrogen-Bond Networks in Water Clusters (H2O)20: An Exhaustive Quantum-Chemical AnalysisCHEMPHYSCHEM, Issue 2 2010Andrei M. Tokmachev Dr. Abstract Water aggregates allow for numerous configurations due to different distributions of hydrogen bonds. The total number of possible hydrogen-bond networks is very large even for medium-sized systems. We demonstrate that targeted ultra-fast methods of quantum chemistry make an exhaustive analysis of all configurations possible. The cage of (H2O)20 in the form of the pentagonal dodecahedron is a common motif in water structures. We calculated the spatial and electronic structure of all hydrogen-bond configurations for three systems: idealized cage (H2O)20 and defect cages with one or two hydrogen bonds broken. More than 3 million configurations studied provide unique data on the structure and properties of water clusters. We performed a thorough analysis of the results with the emphasis on the cooperativity in water systems and the structure-property relations. [source] | |||||||||||||