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Vitaly A Rassolov - One of the best experts on this subject based on the ideXlab platform.
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description of Electronic excited states using Electron Correlation operator
Journal of Chemical Physics, 2013Co-Authors: Bryan Thomas Nichols, Vitaly A RassolovAbstract:The Electron Correlation energy in a chemical system is defined as a difference between the energy of an exact energy for a given Hamiltonian, and a mean-field, or single determinant, approximation to it. A promising way to model Electron Correlation is through the expectation value of a linear two-Electron operator for the Kohn-Sham single determinant wavefunction. For practical reasons, it is desirable for such an operator to be universal, i.e., independent of the positions and types of nuclei in a molecule. The Correlation operator models the effect of Electron Correlation on the interaction energy in a Electron pair. We choose an operator expanded in a small number of Gaussians as a model for Electron Correlation, and test it by computing atomic and molecular adiabatic excited states. The computations are performed within the Δ Self-Consistent Field (ΔSCF) formalism, and are compared to the time-dependent density functional theory model with popular density functionals. The simplest form of the Correlation operator contains only one parameter derived from the helium atom ground state Correlation energy. The Correlation operator approach significantly outperforms other methods in computation of atomic excitation energies. The accuracy of molecular excitation energies computed with the Correlation operator is limited by the shortcomings of the ΔSCF methodology in describing excited states.
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harmonic Electron Correlation operator
Journal of Chemical Physics, 2011Co-Authors: Vitaly A RassolovAbstract:An appealing way to model Electron Correlation within the single determinant wave function formalism is through the expectation value of a linear two-Electron operator. For practical reasons, it is desirable for such an operator to be universal, i.e., not depend on the positions and types of nuclei in a molecule. We show how a perturbation theory applied to a hookium atom provides for a particular form of a Correlation operator, hence called the harmonic Correlation operator. The Correlation operator approach is compared and contrasted to the traditional ways to describe Electron Correlation. To investigate the two-Electron approximation of this operator, we apply it to many-Electron hookium systems. To investigate the harmonic approximation, we apply it to the small atomic systems. Directions of future research are also discussed.
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semiclassical Electron Correlation operator
Journal of Chemical Physics, 2009Co-Authors: Vitaly A RassolovAbstract:The concept of the Correlation operator, introduced 10 years ago as a possible method to model the Electron Correlation effects with single determinant wave functions [Rassolov, J. Chem. Phys. 110, 3672 (1999)], is revisited. We derive a semiclassical limit of the Correlation operator in weakly correlated systems and give its coordinate space representation. Application of this operator to the atomic systems, such as computations of energies of the neutral atoms, energies of the cations, and spin states energy gaps, demonstrates capabilities and limitations of this concept.
Bess Vlaisavljevich - One of the best experts on this subject based on the ideXlab platform.
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multireference Electron Correlation methods journeys along potential energy surfaces
Chemical Reviews, 2020Co-Authors: Jae Woo Park, Rachael Alsaadon, Matthew K Macleod, Toru Shiozaki, Bess VlaisavljevichAbstract:Multireference Electron Correlation methods describe static and dynamical Electron Correlation in a balanced way and, therefore, can yield accurate and predictive results even when single-reference methods or multiconfigurational self-consistent field theory fails. One of their most prominent applications in quantum chemistry is the exploration of potential energy surfaces. This includes the optimization of molecular geometries, such as equilibrium geometries and conical intersections and on-the-fly photodynamics simulations, both of which depend heavily on the ability of the method to properly explore the potential energy surface. Because such applications require nuclear gradients and derivative couplings, the availability of analytical nuclear gradients greatly enhances the scope of quantum chemical methods. This review focuses on the developments and advances made in the past two decades. A detailed account of the analytical nuclear gradient and derivative coupling theories is presented. Emphasis is given to the software infrastructure that allows one to make use of these methods. Notable applications of multireference Electron Correlation methods to chemistry, including geometry optimizations and on-the-fly dynamics, are summarized at the end followed by a discussion of future prospects.
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multireference Electron Correlation methods journeys along potential energy surfaces
arXiv: Chemical Physics, 2019Co-Authors: Jae Woo Park, Rachael Alsaadon, Matthew K Macleod, Toru Shiozaki, Bess VlaisavljevichAbstract:Multireference Electron Correlation methods describe static and dynamical Electron Correlation in a balanced way, and therefore, can yield accurate and predictive results even when single-reference methods or multiconfigurational self-consistent field (MCSCF) theory fails. One of their most prominent applications in quantum chemistry is the exploration of potential energy surfaces (PES). This includes the optimization of molecular geometries, such as equilibrium geometries and conical intersections, and on-the-fly photodynamics simulations; both depend heavily on the ability of the method to properly explore the PES. Since such applications require the nuclear gradients and derivative couplings, the availability of analytical nuclear gradients greatly improves the utility of quantum chemical methods. This review focuses on the developments and advances made in the past two decades. To motivate the readers, we first summarize the notable applications of multireference Electron Correlation methods to mainstream chemistry, including geometry optimizations and on-the-fly dynamics. Subsequently, we review the analytical nuclear gradient and derivative coupling theories for these methods, and the software infrastructure that allows one to make use of these quantities in applications. The future prospects are discussed at the end of this review.
Roland Lindh - One of the best experts on this subject based on the ideXlab platform.
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dynamic Electron Correlation effects on the ground state potential energy surface of a retinal chromophore model
Journal of Chemical Theory and Computation, 2012Co-Authors: Samer Gozem, Mark Huntress, Igor Schapiro, Roland Lindh, Alexander A Granovsky, Celestino Angeli, Massimo OlivucciAbstract:The ground state potential energy surface of the retinal chromophore of visual pigments (e.g., bovine rhodopsin) features a low-lying conical intersection surrounded by regions with variable charge-transfer and diradical Electronic structures. This implies that dynamic Electron Correlation may have a large effect on the shape of the force fields driving its reactivity. To investigate this effect, we focus on mapping the potential energy for three paths located along the ground state CASSCF potential energy surface of the penta-2,4-dieniminium cation taken as a minimal model of the retinal chromophore. The first path spans the bond length alternation coordinate and intercepts a conical intersection point. The other two are minimum energy paths along two distinct but kinetically competitive thermal isomerization coordinates. We show that the effect of introducing the missing dynamic Electron Correlation variationally (with MRCISD) and perturbatively (with the CASPT2, NEVPT2, and XMCQDPT2 methods) leads, invariably, to a stabilization of the regions with charge transfer character and to a significant reshaping of the reference CASSCF potential energy surface and suggesting a change in the dominating isomerization mechanism. The possible impact of such a correction on the photoisomerization of the retinal chromophore is discussed.
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multiconfiguration second order perturbation theory approach to strong Electron Correlation in chemistry and photochemistry
Wiley Interdisciplinary Reviews: Computational Molecular Science, 2012Co-Authors: Daniel Rocasanjuan, Francesco Aquilante, Roland LindhAbstract:Rooted in the very fundamental aspects of many chemical phenomena, and for the majority of photochemistry, is the onset of strongly interacting Electronic configurations. To describe this challenging regime of strong Electron Correlation, an extraordinary effort has been put forward by a young generation of scientists in the development of theories 'beyond' standard wave function and density functional models. Despite their encouraging results, a twenty-and-more-year old approach still stands as the gold standard for these problems: multiconfiguration second-order perturbation theory based on complete active space reference wave function (CASSCF/CASPT2). We will present here a brief overview of the CASSCF/CASPT2 computational protocol, and of its role in our understanding of chemical and photochemical processes.
David A. Mazziotti - One of the best experts on this subject based on the ideXlab platform.
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Exploiting the spatial locality of Electron Correlation within the parametric two-Electron reduced-density-matrix method.
The Journal of chemical physics, 2010Co-Authors: A. Eugene Deprince, David A. MazziottiAbstract:The parametric variational two-Electron reduced-density-matrix (2-RDM) method is applied to computing Electronic Correlation energies of medium-to-large molecular systems by exploiting the spatial locality of Electron Correlation within the framework of the cluster-in-molecule (CIM) approximation [S. Li et al., J. Comput. Chem. 23, 238 (2002); J. Chem. Phys. 125, 074109 (2006)]. The 2-RDMs of individual molecular fragments within a molecule are determined, and selected portions of these 2-RDMs are recombined to yield an accurate approximation to the Correlation energy of the entire molecule. In addition to extending CIM to the parametric 2-RDM method, we (i) suggest a more systematic selection of atomic-orbital domains than that presented in previous CIM studies and (ii) generalize the CIM method for open-shell quantum systems. The resulting method is tested with a series of polyacetylene molecules, water clusters, and diazobenzene derivatives in minimal and nonminimal basis sets. Calculations show that t...
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the cumulant two particle reduced density matrix as a measure of Electron Correlation and entanglement
Journal of Chemical Physics, 2006Co-Authors: Tamas Juhasz, David A. MazziottiAbstract:Several measures of Electron Correlation are compared based on two criteria: (i) the presence of a unique mapping between the reduced variables in the measure and the many-Electron wave function and (ii) the linear scaling of the measure and its variables with system size. We propose the squared Frobenius norm of the cumulant part of the two-particle reduced density matrix (2-RDM) as a measure of Electron Correlation that satisfies these criteria. An advantage of this cumulant-based norm is its ability to measure the Correlation from spin entanglement, which is not contained in the Correlation energy. Alternative measures based on the 2-RDM, such as the von Neumann entropy, do not scale linearly with system size. Properties of the measures are demonstrated with Be, F(2), HF, N(2), and a hydrogen chain.
Stefano Corni - One of the best experts on this subject based on the ideXlab platform.
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proposed alteration of images of molecular orbitals obtained using a scanning tunneling microscope as a probe of Electron Correlation
Physical Review Letters, 2013Co-Authors: Dimitrios Toroz, Massimo Rontani, Stefano CorniAbstract:Scanning tunneling spectroscopy (STS) allows to image single molecules decoupled from the supporting substrate. The obtained images are routinely interpreted as the square moduli of molecular orbitals, dressed by the mean-field Electron-Electron interaction. Here we demonstrate that the effect of Electron Correlation beyond mean field qualitatively alters the uncorrelated STS images. Our evidence is based on the ab-initio many-body calculation of STS images of planar molecules with metal centers. We find that many-body Correlations alter significantly the image spectral weight close to the metal center of the molecules. This change is large enough to be accessed experimentally, surviving to molecule-substrate interactions.
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proposed alteration of images of molecular orbitals obtained using a scanning tunneling microscope as a probe of Electron Correlation
Physical Review Letters, 2013Co-Authors: Dimitrios Toroz, Massimo Rontani, Stefano CorniAbstract:: Scanning tunneling spectroscopy (STS) allows us to image single molecules decoupled from the supporting substrate. The obtained images are routinely interpreted as the square moduli of molecular orbitals, dressed by the mean-field Electron-Electron interaction. Here we demonstrate that the effect of Electron Correlation beyond the mean field qualitatively alters the uncorrelated STS images. Our evidence is based on the ab initio many-body calculation of STS images of planar molecules with metal centers. We find that many-body Correlations alter significantly the image spectral weight close to the metal center of the molecules. This change is large enough to be accessed experimentally, surviving to molecule-substrate interactions.
