202 research outputs found
Linearized Coupled Cluster Correction on the Antisymmetric Product of 1 reference orbital Geminals
We present a Linearized Coupled Cluster (LCC) correction based on an
Antisymmetric Product of 1 reference orbital Geminals (AP1roG) reference state.
In our LCC ansatz, the cluster operator is restricted to double and to single
and double excitations as in standard single-reference CC theory. The
performance of the AP1roG-LCC models is tested for the dissociation of diatomic
molecules (C and F), spectroscopic constants of the uranyl cation
(UO), and the symmetric dissociation of the H hydrogen chain.
Our study indicates that an LCC correction based on an AP1roG reference
function is more robust and reliable than corrections based on perturbation
theory, yielding spectroscopic constants that are in very good agreement with
theoretical reference data.Comment: 9 pages, 4 figure
Dissecting the Bond Formation Process of -Metal-Ethene Complexes with Multireference Approaches
The bonding mechanism of ethene to a nickel or palladium center is studied by
the density matrix renormalization group algorithm, the complete active space
self consistent field method, coupled cluster theory, and density functional
theory. Specifically, we focus on the interaction between the metal atom and
bis-ethene ligands in perpendicular and parallel orientations. The bonding
situation in these structural isomers is further scrutinized using energy
decomposition analysis and quantum information theory. Our study highlights the
fact that when two ethene ligands are oriented perpendicular to each other, the
complex is stabilized by the metal-to-ligand double-back-bonding mechanism.
Moreover, we demonstrate that nickel-ethene complexes feature a stronger and
more covalent interaction between the ligands and the metal center than
palladium-ethene compounds with similar coordination spheres.Comment: 13 pages, 9 figure
Efficient description of strongly correlated electrons with mean-field cost
We present an efficient approach to the electron correlation problem that is
well-suited for strongly interacting many-body systems, but requires only
mean-field-like computational cost. %which is based on orbital optimization of
electron pairs. The performance of our approach is illustrated for the
one-dimensional Hubbard model with periodic boundary conditions for different
chain lengths, and for the non-relativistic quantum chemical Hamiltonian
exploring the symmetric dissociation of the H hydrogen chain.Comment: 4 pages, 4 figure
Melting dynamics of large ice balls in a turbulent swirling flow
We study the melting dynamics of large ice balls in a turbulent von Karman
flow at very high Reynolds number. Using an optical shadowgraphy setup, we
record the time evolution of particle sizes. We study the heat transfer as a
function of the particle scale Reynolds number for three cases: fixed ice balls
melting in a region of strong turbulence with zero mean flow, fixed ice balls
melting under the action of a strong mean flow with lower fluctuations, and ice
balls freely advected in the whole flow. For the fixed particles cases, heat
transfer is observed to be much stronger than in laminar flows, the Nusselt
number behaving as a power law of the Reynolds number of exponent 0.8. For
freely advected ice balls, the turbulent transfer is further enhanced and the
Nusselt number is proportional to the Reynolds number. The surface heat flux is
then independent of the particles size, leading to an ultimate regime of heat
transfer reached when the thermal boundary layer is fully turbulent
Tensor Product Approximation (DMRG) and Coupled Cluster method in Quantum Chemistry
We present the Copupled Cluster (CC) method and the Density matrix
Renormalization Grooup (DMRG) method in a unified way, from the perspective of
recent developments in tensor product approximation. We present an introduction
into recently developed hierarchical tensor representations, in particular
tensor trains which are matrix product states in physics language. The discrete
equations of full CI approximation applied to the electronic Schr\"odinger
equation is casted into a tensorial framework in form of the second
quantization. A further approximation is performed afterwards by tensor
approximation within a hierarchical format or equivalently a tree tensor
network. We establish the (differential) geometry of low rank hierarchical
tensors and apply the Driac Frenkel principle to reduce the original
high-dimensional problem to low dimensions. The DMRG algorithm is established
as an optimization method in this format with alternating directional search.
We briefly introduce the CC method and refer to our theoretical results. We
compare this approach in the present discrete formulation with the CC method
and its underlying exponential parametrization.Comment: 15 pages, 3 figure
Projected seniority-two orbital optimization of the Antisymmetric Product of one-reference orbital Geminal
We present a new, non-variational orbital-optimization scheme for the
Antisymmetric Product of one-reference orbital Geminal wave function. Our
approach is motivated by the observation that an orbital-optimized
seniority-zero configuration interaction (CI) expansion yields similar results
to an orbital-optimized seniority-zero-plus-two CI expansion [J. Chem. Phys.,
135, 044119 (2011)]. A numerical analysis is performed for the C, LiF and
CH molecules as well as for the symmetric stretching of hypothetical
(linear) hydrogen chains. For these test cases, the proposed
orbital-optimization protocol yields similar results to its variational orbital
optimization counterpart, but prevents symmetry-breaking of molecular orbitals
in most cases.Comment: 7 pages, 2 figure
Entanglement Measures for Single- and Multi-Reference Correlation Effects
Electron correlation effects are essential for an accurate ab initio
description of molecules. A quantitative a priori knowledge of the single- or
multi-reference nature of electronic structures as well as of the dominant
contributions to the correlation energy can facilitate the decision regarding
the optimum quantum chemical method of choice. We propose concepts from quantum
information theory as orbital entanglement measures that allow us to evaluate
the single- and multi-reference character of any molecular structure in a given
orbital basis set. By studying these measures we can detect possible artifacts
of small active spaces.Comment: 14 pages, 4 figure
Photon scattering errors during stimulated Raman transitions in trapped-ion qubits
We study photon scattering errors in stimulated Raman driven quantum logic
gates. For certain parameter regimes, we find that previous, simplified models
of the process significantly overestimate the gate error rate due to photon
scattering. This overestimate is shown to be due to previous models neglecting
the detuning dependence of the scattered photon frequency and Lamb-Dicke
parameter, a second scattering process, interference effects on scattering
rates to metastable manifolds, and the counter-rotating contribution to the
Raman transition rate. The resulting improved model shows that there is no
fundamental limit on gate error due to photon scattering for electronic ground
state qubits in commonly-used trapped-ion species when the Raman laser beams
are red detuned from the main optical transition. Additionally, photon
scattering errors are studied for qubits encoded in metastable
manifold, showing that gate errors below are achievable for all
commonly-used trapped ions.Comment: 24 pages, 8 figures, to be submitted to Phys. Rev. A. In this
version, we changed the two-qubit gate under consideration. Originally, we
considered a gate driven by two perpendicular pairs of Raman beams. In this
version, we consider a gate driven by a pair of Raman beams
counterpropagating against a third Raman bea
Acceptor binding energies in GaN and AlN
We employ effective mass theory for degenerate hole-bands to calculate the
acceptor binding energies for Be, Mg, Zn, Ca, C and Si substitutional acceptors
in GaN and AlN. The calculations are performed through the 66
Rashba-Sheka-Pikus and the Luttinger-Kohn matrix Hamiltonians for wurtzite (WZ)
and zincblende (ZB) crystal phases, respectively. An analytic representation
for the acceptor pseudopotential is used to introduce the specific nature of
the impurity atoms. The energy shift due to polaron effects is also considered
in this approach. The ionization energy estimates are in very good agreement
with those reported experimentally in WZ-GaN. The binding energies for ZB-GaN
acceptors are all predicted to be shallower than the corresponding impurities
in the WZ phase. The binding energy dependence upon the crystal field splitting
in WZ-GaN is analyzed. Ionization levels in AlN are found to have similar
`shallow' values to those in GaN, but with some important differences, which
depend on the band structure parameterizations, especially the value of crystal
field splitting used.Comment: REVTEX file - 1 figur
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