VB participants talks abstract page
Short talks will be given by volunteer participants. They should have a direct connection with VB theory. They may present, for instance, a recent research work connected with VB theory, a chemical problem on which a VB method or model could be relevant, or a method/algorithmic idea or scheme which could be of interest to the VB community. Presentation of open questions that could lead to discussions is welcome. Talks should be as compact and "straight to the point" as possible, and they should leave ample space for ideas exchange and discussions among the participants.
SPEAKERS : please add below, in your own section, your title talk and abstract :
- first : log in (see also : How to create an account) ;
- click on your name in the "Contents" box below, this will lead you to your own section ;
- your section starts with your name as the title line, click on [edit] (far right) ;
- To insert a file in your abstract, you will have to :
- upload your file (see : How to upload files on the wiki) ;
- then to insert the file in your text (see : How to insert files onto the wiki).
Duration of the talks will be 30 minutes including questions, so we recommend a <20 minutes talk in order to give space for discussions.
The order of abstracts follow the alphabetic order.
Celestino Angeli
Dipartimento di Chimica, Via Borsari 46, 44121 Ferrara (ITALY), email
VB reading of CASSCF wavefunctions: a few examples for ground and excited states.
A few recent studies in which the VB reading of CASSCF wavefunctions has been used to describe the electronic nature of ground and excited states of small and medium size molecules will be presented. Moreover, it will be shown how this analysis has allowed to unravel the origin of the peculiar difficulties found in the description of some systems, such as, for instance, the ionic excited states of conjugated molecules. In some cases, such a comprehension has enabled us to devise efficient MO-based computational approaches.
References
1] J.P. Malrieu, N. Guihery, C.J. Calzado, C. Angeli, Bond electron pair: its relevance and analysis from the quantum chemistry point of view, J. Comp. Chem., 28(1), 35-50, (2007).
[2] C. Angeli, R. Cimiraglia, J.P. Malrieu, On the relative merits of non-orthogonal and orthogonal Valence Bond methods illustrated on the hydrogen molecule, J. Chem. Ed., 85(1), 150-158, (2008).
[3] C. Angeli, J.P. Malrieu, Aromaticity: an ab-initio evaluation of the properly cyclic delocalization energy and of the <math>\pi</math>-delocalization energy distortivity in benzene, J. Phys. Chem. A, 112(45), 11481-11486, (2008).
[4] C. Angeli, On the nature of the <math>\pi\rightarrow\pi^*</math> ionic excited states: the V state of ethene as a prototype, J. Comp. Chem., 30(8), 1319-1333, (2009).
[5] C. Angeli, An analysis of the dynamic <math>\sigma</math> polarization in the V state of ethene, Int. J. Quant. Chem ., 110(13), 2436-2447, (2010).
Samuel De Visser
My address, [1]
Enzymatic oxygen atom transfer reactions: Trends explained with Valence Bond Theory
Text of the abstract.
References
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Jiali Gao
Multistate Density Functional Theory with Block-localized Configurations
In this paper, I will describe a multistate density functional theory (MSDFT) [1], in which the individual valence bond-like or diabatic states are represented through block-localization of Kohn-Sham orbitals[1,2]. The method is an extension of the mixed molecular orbital and valence bond (MOVB) method introduced by Yirong Mo and myself [3]. In MSDFT, dynamic correlation effects are incorporated into the localized diabatic configurations using BL-DFT. The multiconfiguration character of MSDFT, involving nonorthogonal determinant functions, consists of static correlation contributions. We try to discuss the issue of possible double counting correlation effects through model calculations [4]. Some specific applications will also be discussed including proton-coupled electron transfer and singlet fission.
References
[1] Cembran, J.; Song, L.; Mo, Y.; Gao, J. J. Chem. Theory Comput, 2009, 5, 2702.
[2] Mo, Y.; Song, L.; Lin, Y. J. Phys. Chem. A 2007, 111, 8291.
[3] Y. Mo, J. Gao, J. Phys. Chem. A, 2000, 104, 3012.
[4] Y. Mo, P. Bao, J. Gao,Phys. Chem. Chem. Phys., 2011, 13, 6760.
Lynn Kamerlin
Address, email
Paradynamics: Harnessing Reference Potentials to Accelerate Ab Initio QM/MM Free-Energy Calculations
N. Plotnikov1, Shina C. L. Kamerlin2 and A. Warshel1
- Department of Chemistry, University of Southern California, USA
- Department of Cell and Molecular Biology (ICM), Uppsala University, Sweden
Ab initio QM/MM (QM(ai)/MM) calculations have shown tremendous promise as a tool with which to obtain reaction free energies in the condensed phase or in more complex biological systems. Despite their power, however, performing sufficient sampling for such calculations to be reliable in a computationally tractable manner remains a significant challenge, and various approaches are currently being developed to address this problem. We present here an updated version of the idea of using a classical reference potential for QM(ai)/MM calculations, which we refer to as “Paradynamics”. This approach is based on iteratively refining an empirical valence bond (EVB) potential until it is as close as possible to the actual ab initio potential, using a semi-automated refinement procedure. The refined EVB potential can then be used to construct the actual corresponding free-energy profile, and any remaining difference between the two potentials can be evaluated using either free energy perturbation or the linear response approximation, and added to the obtained EVB activation barrier as a correction. We demonstrate the efficacy of this approach using a simple test reaction, and provide benchmarks to illustrate the significant reduction of computational cost it provides. We also note that this is a broadly generalized approach that is not limited to free energy calculations, but can be utilized to address a range of problems where it is necessary to explore complex energy landscapes.
Padeleimon Karafiloglou
Address, [2]
POLY-ELECTRON POPULATION ANALYSIS OF MO WAVEFUNCTIONS: A ‘theoretical microscope’ to explore local VB-type structures. (A proposition to extend this analysis for VB wavefunctions, in order to explore molecular fragments, as for example ‘Functional Groups’)
Abstract...
References
[1] P. Karafiloglou, J. Chem. Phys. 130 (2009) 164103
[2] P. Papanikolaou, P. Karafiloglou, J. Phys. Chem. A 112 (2008) 8839
[3] P. Karafiloglou, J. Phys. Chem. A 105 (2001)4524
[4] P. Papanikolaou, P. Karafiloglou Chem. Phys. 342(2007)288
Christine Lepetit
Address, email
A three-level model for two-photon absorption. A combined Valence Bond and Electron Localization Function approach.
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Jules Tshishimbi Muya
Address, email
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Josep M. Oliva
Molecular Architectures built from Heteroborane Clusters: Electronic Structure and Beyond
In the last decade we have been interested in the electronic structure of heteroborane clusters [1], both in their groundstates and excited states. One of the challenges here is to build molecular architectures from heteroborane clusters in different dimensions and predict their shapes (geometries) and the electronic structures of their groundstates and low-lying states [2]. For instance, one can connect an S=1/2 heteroborane cage - CB(11)H(12)(·) - into an (in)finite 1D linear chain and map the electronic structure results onto a Heisenberg spin Hamiltonian [3]. Finally a VB interpretation of the many-electron problem within this field of research would be desirable [4].
References
[1] J.M. Oliva, N.L. Allan, P.v.R. Schleyer, C. Viñas, F. Teixidor, J. Am. Chem. Soc. 127 (2005) 13538-13547
[2] J.M. Oliva, D.J. Klein, P.v.R. Schleyer, L. Serrano-Andrés, Pure Appl. Chem. 81 (2009) 719-729
[3] J.M. Oliva, Adv. Quantum Chem., in press
[4] D.J. Klein, J.M. Oliva, Int. J. Quantum Chem. 110 (2010) 2784-2800
Carol Parish
Department of Chemistry, University of Richmond, email
Characterizing Heteroaromatic Diradicals
We are interested in the pyrolysis and combustion reactions of the molecular constituents of oil shale. Oil shale contains asphaltenes, a complex polydisperse molecular mixture, containing fused aromatic cores with long chain alkyl arms. In the high temperature regime, hydrogen abstraction reactions are possible forming heteroaromatic diradicals. We are in the process of characterizing properly diradical asphaltene model compounds – the diradicals of benzene, thiophene, fulvene, pyrrole and furan. A molecular orbital based approach will be presented along with preliminary results. Going forward, we would like to apply ab initio Valence Bond methods to the characterization of these molecules.
References
[1] An Extended Multireference Study of the Electronic States of para-benzyne, Evan Wang, Carol Parish and Hans Lischka, Journal of Chemical Physics, 2008 129, 44306:1-44306:8
Mario Piris
Address, email
The Molecular Orbitals in Natural Orbital Functional Theory
One-electron pictures have long helped to our understanding of chemical bonding. It has recently been pointed out that natural orbital functional theory (NOFT) can provide an orbital picture in agreement with the chemical intuition [1,2]. The NOFT appeared few decades ago [3,4]. An approximate NOF requires a reconstruction of the two-particle reduced density matrix (2-RDM) in terms of the one-particle reduced density matrix (1-RDM). Such reconstruction of the 2-RDM has been achieved using the cumulant expansion leading to the Piris NOF (PNOF) [5]. The PNOF is based on an explicit ansatz of the two-particle cumulant λ(Δ,Π) satisfying necessary N-representability conditions for the 2-RDM. Appropriate forms of matrices ∆ and Π lead to different implementations PNOFi (i=1,5) [6]. In this presentation, the theory behind the PNOF5 is briefly outlined. Some examples of strongly correlated systems, where density functionals yield pathological failures, are presented to illustrate the potentiality of the NOFT.
In this talk, we show that PNOF5 leads to two complementary representations of the one-electron picture in molecules, namely, the natural orbital (NO) representation and the canonical orbital (CO) representation. The PNOF5 NO representation leads generally to the localization of the molecular orbitals. Accordingly, it provides an orbital picture that agrees closely with the empirical valence shell electron pair repulsion theory (VSEPR) and the Bent's rule, along with the theoretical valence bond (VB) method. On the other hand, the equivalent CO representation, obtained from the diagonalization of the matrix of Lagrange multipliers, affords delocalized molecular orbitals adapted to the symmetry of the molecule. We show that the one-particle energies associated to the COs can yield reasonable principal ionization potentials. The relationship between NOs and COs is illustrated by several examples, showing that both orbital representations complement each other.
The calculations were carried out with our implementation, the PNOFID code [7], based on a recent proposed algorithm [8].
References
[1] J. M. Matxain, M. Piris, J. Uranga, X. Lopez, G. Merino, J. M. Ugalde, "The Nature of the Chemical Bonds from PNOF5 calculations", ChemPhysChem. DOI: 10.1002/cphc.201200205 (2012).
[2] J. M. Matxain, M. Piris, J. M. Mercero, X. Lopez, J. M. Ugalde, "sp3 hybrid orbitals and ionization energies of methane from PNOF5", Chem. Phys. Lett. 531, 272 (2012).
[3] T. L. Gilbert, Phys. Rev. B 12, 2111 (1975); M. Levy, Proc. Natl. Acad. Sci. U.S.A. 76, 6062 (1979); S. M. Valone, J. Chem. Phys. 73, 1344 (1980).
[4] M. Piris, "Natural Orbital Functional Theory" in Reduced-Density-Matrix Mechanics: With Applications to Manyelectron Atoms and Molecules, edited by D. A. Mazziotti, Adv. Chem. Phys. 134, 387 (2007), and references therein.
[5] M. Piris, "A new approach for the Two-Electron Cumulant in Natural Orbital Functional theory", Int. J. Quantum Chem. 106, 1093 (2006).
[6] M. Piris, "A natural orbital functional based on an explicit approach of the two-electron cumulant", Int. J. Quantum Chem. DOI: 10.1002/qua.24020 (2012).
[7] M. Piris, PNOFID, downloadable at http://www.ehu.es/mario.piris/#Software.
[8] M. Piris, J. M. Ugalde, "Iterative diagonalization for orbital optimization in the Natural Orbital Functional Theory", J. Comp, Chem. 30, 2078 (2009).
Romain Ramozzi
Address, email
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Avital Shurki
Address, email
Valence Bond Insights on Enzyme Catalysis
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Matthias Stein
Address, email
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Tom Ziegler
Analyzing Complex Electronic Structure Calculations on Large Molecules in Simple Chemical Terms
In this talk we shall introduce a new scheme for chemical bond analysis [J. Chem. Theory Comput., 2009] by combining the Extended Transition State (ETS) method [Theor.Chim.Acta 1977,46,1] with the Natural Orbitals for Chemical Valence (NOCV) theory [J.Phys.Chem.A. 2008,112,1933]. The ETS-NOCV charge and energy decomposition scheme makes it not only possible to decompose the deformation density, Δρ, into the different components (such as σ,π,δ etc.) of the chemical bond, but it also provides the corresponding energy contributions to the total bond energy from these components. Thus, the ETS-NOCV scheme offers a compact, qualitative and quantitative, picture of the chemical bond formation within one common theoretical framework. Although, the ETS-NOCV approach contains a certain arbitrariness in the definition of the molecular subsystems that constitute the whole molecule, it can be widely used for the description of different types of chemical bonds. The applicability of the ETS-NOCV scheme is demonstrated for single (H3X-XH3, for X = C, Si, Ge, Sn) and multiple (H2X=XH2, H3CXXCH3, for X = C, Ge) covalent bonds between main group elements, for sextuple and quadruple bonds between metal centers (Cr2, Mo2, W2, [Cl4CrCrCl4]4-) and for double bonds between a metal and a main group element ((CO)5Cr=XH2, for X = C, Si, Ge, Sn). Applications are also given to hydrogen- and agostic bonds as well as the interaction between adsorbates and metal surfaces. The scheme is finally used to explain the trans-effect in square planar platimum complexes.
References
[1] Mariusz P. Mitoraj, Artur Michalak and Tom Ziegler “A Combined Charge and Energy Decomposition Scheme for Bond Analysis” J. Chem. Theory Comput., 2009,5 , 962–975