1,721,094 research outputs found
Poster P15: Excited states of molecular solutes with Quantum Monte Carlo: vertical transition and geometry optimization
No full textRecently, we have developed a novel Polarizable Continuum Model (PCM), which includes both surface and volume polarization of the dielectric medium (pure SVPE scheme), designed for the Quantum Monte Carlo (QMC) treatment of the solute. In particular, the treatment of volume polarization, due to quantum mechanical penetration of the solute charge density in the solvent domain, is based on quantum Monte Carlo techniques. The method allows to accurately solve Poisson's equation of the solvation model coupled with the Schrödinger equation for the solute [1,2,3]. The present model has been now extended to treat the effects of solvation in solute vertical electronic transitions and to the search of the solute equilibrium geometry in the excited states. For the first case, here we show the results of our study
performed on fast n → pi* and pi → pi* vertical transitions of s-trans- acrolein in water [4]. To perform calculations in a non-equilibrium solvation regime for the solute excited state, we have added a correction to the global dielectric polarization charge density, obtained self consistently with the solute ground-state wave function by assuming a linear-response scheme. The calculated solvatochromic shifts are properly described. For the second case, we start from recent
advances made to carry out the ground- and excited-state geometry optimization within QMC [5]. For the present purpose, we have extended the calculation of the forces to include solvent e_ects through our QMC implementation of PCM [6]. We show results, performed at the variational Monte Carlo level, on the excited-state geometry optimization of some small organic molecules in water solution and we make a comparison with the more widely used TDDFT and CASPT2 methods.
[1] C. Amovilli, C. Filippi, F. M. Floris, J. Phys. Chem. B (2006) 110 26225.
[2] C. Amovilli, C. Filippi, F. M. Floris, J. Chem. Phys. (2008) 129 244106.
[3] F. M. Floris, C. Filippi, C. Amovilli, J. Chem. Phys. (2012) 137 075102.
[4] F. M. Floris, C. Filippi, C. Amovilli, J. Chem. Phys. (2014) 140 034109.
[5] R. Guareschi, C. Filippi, J. Chem. Theor. Comput. (2013) 9 5513.
[6] R. Guareschi, F. M. Floris, C. Amovilli, C. Filippi, in preparation (2014)
Preemptive Scheduling on Unrelated Parallel Machines with Sequence Setups and High Multiplicity: Heuristic Approaches
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
THE LINEAR BALANCING FLOW PROBLEM
No full textThe original motivation for investigating the Linear Balancing Flow Problem (LBFP) came from the optimization of a real-life production plan. Each instance of LBFP is a linear programming problem and can be interpreted as a flow problem on a bipartite network with gains. In the literature the typical flow problem on networks with gains is either a max flow or a min cost flow problem. Here we consider a different kind of objective, namely, how to optimally balance the flow out of a given set of nodes. An original algorithm is suggested which takes advantage of the particular problem structure. Theoretically it may be viewed as a specialized version of the Simplex. However it is more efficient than the latter: in fact, it requires much less memory and computing time. The key feature consists in associating a tree to the set of variables of the dual problem that are out of the current basis. The justification of the proposed algorithm does not immediately follow from that of the Simplex. A direct proof of its validity is provided and simple numerical examples are reported
Excitation energies of retinal chromophores: critical role of the structural model
No full textWe employ a variety of highly-correlated approaches including quantum Monte Carlo (QMC) and the n-electron valence state perturbation theory (NEVPT2) to compute the vertical excitation energies of retinal protonated Schiff base (RPSB) models in the gas phase. We find that the NEVPT2 excitation energies are in good agreement with the QMC values and confirm our previous findings that the complete-active-space perturbation (CASPT2) approach yields accurate excitations for RPSB models only when the more recent zero-order IPEA Hamiltonian is employed. The excitations computed with the original zero-order formulation of CASPT2 are instead systematically red-shifted by more than 0.3 eV. We then focus on the full 11-cis retinal chromophore and show that the M06-2X and MP2 approaches provide reliable ground-state equilibrium structures for this system while the complete-active-space self-consistent field (CASSCF) geometry is characterized by significantly higher ground-state energies at the NEVPT2 and CASPT2 level. Our calibration of the structural model together with the general agreement of all highly-correlated excited-state methods allows us to reliably assign a value of about 2.3 eV to the vertical excitation of 11-cis RPSB in the gas-phase
Coupling Quantum Monte Carlo to a Nonlinear Polarizable Continuum Model for Spherical Solutes
No full textStarting from the nonlinear dielectric response model of Sandberg and Edholm, we derive an analytical expression of the polarization contribution to the solvation free energy in terms of the electronic density of the solute and the dielectric properties of the solvent. The solvent inhomogeneity is taken into account with the use of a smooth switching function whose spacial variation is established on the basis of how the solvent is arranged around the solute. An explicit form of a local potential representing the solvent effect on the solute is thus obtained by functional analysis. This effective potential can be combined with density functional or quantum chemical methods for the quantum mechanical treatment of the solute. Here, we use quantum Monte Carlo techniques for the solute and apply the method to the hydration of atomic ions finding very good agreement with experimental data
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