The Experts below are selected from a list of 60 Experts worldwide ranked by ideXlab platform
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.
Emily A Carter - One of the best experts on this subject based on the ideXlab platform.
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periodic density functional embedding theory for complete active space self consistent field and configuration interaction calculations ground and excited states
Journal of Chemical Physics, 2002Co-Authors: Thorsten Kluner, Niranjan Govind, Yan Alexander Wang, Emily A CarterAbstract:We extend our recently reported embedding theory [J. Chem. Phys. 110, 7677 (1999)] to calculate not only improved descriptions of ground states, but now also localized excited states in a periodically infinite condensed phase. A local region of the solid is represented by a small cluster for which high quality quantum chemical calculations are performed. The interaction of the cluster with the extended condensed phase is taken into account by an effective embedding potential. This potential is calculated by periodic density functional theory (DFT) and is used as a one-Electron Operator in subsequent cluster calculations. Among a variety of benchmark calculations, we investigate a CO molecule adsorbed on a Pd(111) surface. By performing complete active space self-consistent field, configuration interaction (CI), and Moller–Plesset perturbation theory of order n (MP-n), we not only were able to obtain accurate adsorption energies via local corrections to DFT, but also vertical excitation energies for an int...
Frank Neese - One of the best experts on this subject based on the ideXlab platform.
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efficient and accurate approximations to the molecular spin orbit coupling Operator and their use in molecular g tensor calculations
Journal of Chemical Physics, 2005Co-Authors: Frank NeeseAbstract:Approximations to the Breit-Pauli form of the spin-orbit coupling (SOC) Operator are examined. The focus is on approximations that lead to an effective quasi-one-Electron Operator which leads to efficient property evaluations. In particular, the accurate spin-orbit mean-field (SOMF) method developed by Hess, Marian, Wahlgren, and Gropen is examined in detail. It is compared in detail with the “effective potential” spin-orbit Operator commonly used in density functional theory (DFT) and which has been criticized for not including the spin-other orbit (SOO) contribution. Both Operators contain identical one-Electron and Coulomb terms since the SOO contribution to the Coulomb term vanishes exactly in the SOMF treatment. Since the DFT correlation functional only contributes negligibly to the SOC the only difference between the two Operators is in the exchange part. In the SOMF approximation, the SOO part is equal to two times the spin-same orbit contribution. The DFT exchange contribution is of the wrong sign...
Subir Sachdev - One of the best experts on this subject based on the ideXlab platform.
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spin density wave order topological order and fermi surface reconstruction
Physical Review B, 2016Co-Authors: Subir Sachdev, Shubhayu Chatterjee, Erez Berg, Yoni SchattnerAbstract:In the conventional theory of density wave ordering in metals, the onset of spin density wave (SDW) order coincides with the reconstruction of the Fermi surfaces into small ``pockets.'' We present models which display this transition, while also displaying an alternative route between these phases via an intermediate phase with topological order, no broken symmetry, and pocket Fermi surfaces. The models involve coupling emergent gauge fields to a fractionalized SDW order, but retain the canonical Electron Operator in the underlying Hamiltonian. We establish an intimate connection between the suppression of certain defects in the SDW order and the presence of Fermi surface sizes distinct from the Luttinger value in Fermi liquids. We discuss the relevance of such models to the physics of the hole-doped cuprates near optimal doping.
Tetsuo Matsui - One of the best experts on this subject based on the ideXlab platform.
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the t j model of hard core bosons in slave particle representation and its monte carlo simulations
Journal of Physics: Conference Series, 2012Co-Authors: Yuki Nakano, Takumi Ishima, Naohiro Kobayashi, Ikuo Ichinose, Kazuhiko Sakakibara, Tetsuo MatsuiAbstract:We study the system of hard-core bosons (HCB) with two species in the three-dimensional lattice at finite temperatures. In the strong-correlation limit, the system becomes the bosonic t-J model, that is, the t-J model of "bosonic Electrons". The bosonic "Electron" Operator Bxσ at the site x with a two-component spin σ(= 1, 2***) is treated as a HCB Operator, and represented by a composite of two slave particles; a spinon described by a Schwinger boson (CP1 boson) zxσ and a holon described by a HCB field x as Bxσ = †xzxσ.*** This x is again represented by another CP1 quasi-spinon Operator ωxa*** (a = 1, 2***). The phase diagrams of the resulting double CP1 system obtained by Monte Carlo simulations involve first-order and second-order phase boundaries. We present in detail the techniques and algorithm to reduce the hysteresis and locate the first-order transition points.
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finite temperature phase diagram of two component bosons in a cubic optical lattice three dimensional t j model of hard core bosons
Physical Review A, 2012Co-Authors: Yuki Nakano, Takumi Ishima, Naohiro Kobayashi, Takahiro Yamamoto, Ikuo Ichinose, Tetsuo MatsuiAbstract:We study the three-dimensional bosonic $t$-$J$ model, i.e., the $t$-$J$ model of ``bosonic Electrons,'' at finite temperatures. This model describes the $s=\frac{1}{2}$ Heisenberg spin model with the anisotropic exchange coupling ${J}_{\ensuremath{\perp}}=\ensuremath{\alpha}{J}_{z}$ and doped bosonic holes, which is an effective system of the Bose-Hubbard model with strong repulsions. The bosonic ``Electron'' Operator ${B}_{r\ensuremath{\sigma}}$ at the site $r$ with a two-component (pseudo)spin $\ensuremath{\sigma}(=1,2)$ is treated as a hard-core boson Operator. By means of Monte Carlo simulations, we study its finite-temperature phase structure including the $\ensuremath{\alpha}$ dependence, the possible phenomena-like appearance of checkerboard long-range order, supercounterflow, superfluidity, phase separation, etc. The obtained results, which clarify the relation between various phases, may be taken as predictions about experiments of two-component cold bosonic atoms in a cubic optical lattice.