1,692 research outputs found
North-South Dialogues in Forced Migration
The following speech was given by Elizabeth McWeeny, President of the Canadian Council of Refugees, on the occasion of the opening of the tenth biennial conference of the International Association of the Study of Forced Migration, hosted by the Centre for Refugee Studies of York University in June 2006
Ab initio Wannier-function-based many-body approach to Born charge of crystalline insulators
In this paper we present an approach aimed at performing many-body
calculations of Born-effective charges of crystalline insulators, by including
the electron-correlation effects. The scheme is implemented entirely in the
real space, using Wannier-functions as single-particle orbitals. Correlation
effects are computed by including virtual excitations from the Hartree-Fock
mean field, and the excitations are organized as per a Bethe-Goldstone-like
many-body hierarchy. The results of our calculations suggest that the approach
presented here is promising.Comment: 5 pages, to appear in Phys. Rev. B. (Rapid Comm., Dec 15, 2004
Ab initio Wannier-function-based correlated calculations of Born effective charges of crystalline LiO and LiCl
In this paper we have used our recently developed ab initio
Wannier-function-based methodology to perform extensive Hartree-Fock and
correlated calculations on LiO and LiCl to compute their Born effective
charges. Results thus obtained are in very good agreement with the experiments.
In particular, for the case of LiO, we resolve a controversy originating
in the experiment of Osaka and Shindo {[}Solid State Commun. 51 (1984) 421] who
had predicted the effective charge of Li ions to be in the range 0.58--0.61, a
value much smaller compared to its nominal value of unity, thereby, suggesting
that the bonding in the material could be partially covalent. We demonstrate
that effective charge computed by Osaka and Shindo is the Szigeti charge, and
once the Born charge is computed, it is in excellent agreement with our
computed value. Mulliken population analysis of LiO also confirms ionic
nature of the bonding in the substance.Comment: 11 pages, 1 figure. To appear in Phys. Rev. B (Feb 2008
The Bravyi-Kitaev transformation for quantum computation of electronic structure
Quantum simulation is an important application of future quantum computers
with applications in quantum chemistry, condensed matter, and beyond. Quantum
simulation of fermionic systems presents a specific challenge. The
Jordan-Wigner transformation allows for representation of a fermionic operator
by O(n) qubit operations. Here we develop an alternative method of simulating
fermions with qubits, first proposed by Bravyi and Kitaev [S. B. Bravyi, A.Yu.
Kitaev, Annals of Physics 298, 210-226 (2002)], that reduces the simulation
cost to O(log n) qubit operations for one fermionic operation. We apply this
new Bravyi-Kitaev transformation to the task of simulating quantum chemical
Hamiltonians, and give a detailed example for the simplest possible case of
molecular hydrogen in a minimal basis. We show that the quantum circuit for
simulating a single Trotter time-step of the Bravyi-Kitaev derived Hamiltonian
for H2 requires fewer gate applications than the equivalent circuit derived
from the Jordan-Wigner transformation. Since the scaling of the Bravyi-Kitaev
method is asymptotically better than the Jordan-Wigner method, this result for
molecular hydrogen in a minimal basis demonstrates the superior efficiency of
the Bravyi-Kitaev method for all quantum computations of electronic structure
Antiferromagnetic Exchange Interaction between Electrons on Degenerate LUMOs in Benzene Dianion
We discuss the ground state of Benzene dianion (Bz) on the basis of
the numerical diagonalization method of an effective model of orbitals.
It is found that the ground state can be the spin singlet state, and the
exchange coupling between LUMOs can be antiferromagnetic.Comment: Accepted for publication in J. Phys. Soc. Jpn., 2 pages, 3 figure
A Diabatic Three-State Representation of Photoisomerization in the Green Fluorescent Protein Chromophore
We give a quantum chemical description of bridge photoisomerization reaction
of green fluorescent protein (GFP) chromophores using a representation over
three diabatic states. Bridge photoisomerization leads to non-radiative decay,
and competes with fluorescence in these systems. In the protein, this pathway
is suppressed, leading to fluorescence. Understanding the electronic structure
of the photoisomerization is a prerequisite to understanding how the protein
suppresses this pathway and preserves the emitting state of the chromophore. We
present a solution to the state-averaged complete active space problem, which
is spanned at convergence by three fragment-localized orbitals. We generate the
diabatic-state representation by applying a block diagonalization
transformation to the Hamiltonian calculated for the anionic chromophore model
HBDI with multi-reference, multi-state perturbation theory. The diabatic states
that emerge are charge-localized structures with a natural valence-bond
interpretation. At planar geometries, the diabatic picture recaptures the
charge transfer resonance of the anion. The strong S0-S1 excitation at these
geometries is reasonably described within a two-state model, but extension to a
three-state model is necessary to describe decay via two possible pathways
associated with photoisomerization of the (methine) bridge. Parametric
Hamiltonians based on the three-state ansatz can be fit directly to data
generated using the underlying active space. We provide an illustrative example
of such a parametric Hamiltonian
Closed-form expressions for correlated density matrices: application to dispersive interactions and example of (He)2
Empirically correlated density matrices of N-electron systems are
investigated. Exact closed-form expressions are derived for the one- and
two-electron reduced density matrices from a general pairwise correlated wave
function. Approximate expressions are proposed which reflect dispersive
interactions between closed-shell centro-symmetric subsystems. Said expressions
clearly illustrate the consequences of second-order correlation effects on the
reduced density matrices. Application is made to a simple example: the (He)2
system. Reduced density matrices are explicitly calculated, correct to second
order in correlation, and compared with approximations of independent electrons
and independent electron pairs. The models proposed allow for variational
calculations of interaction energies and equilibrium distance as well as a
clear interpretation of dispersive effects on electron distributions. Both
exchange and second order correlation effects are shown to play a critical role
on the quality of the results.Comment: 22 page
Weak Interactions Between Molecules
Explicit formulae are derived for the calculation of dispersion
energies between large molecules, at various levels of approximmion.
The derivation introduces frequency-dependent polarizabilities
(FDPs), whiCh describe the propagation of electron density fluctuat-·
ions within each of the separate molecules, but avoids the usual
multipole expansion. The resultant dispersion energy formula reveals
the presence of long-range (R-2) energy terms between moms
of the different molecules and provides a basis for semi-empirical
models based on pairwise atom-atom interactions.
A rapidly convergent SCF procedure for calculating the required
FDPs is also described
- …