5,375 research outputs found
Generalized Unitary Coupled Cluster Wavefunctions for Quantum Computation
We introduce a unitary coupled-cluster (UCC) ansatz termed -UpCCGSD that
is based on a family of sparse generalized doubles (D) operators which provides
an affordable and systematically improvable unitary coupled-cluster
wavefunction suitable for implementation on a near-term quantum computer.
-UpCCGSD employs products of the exponential of pair coupled-cluster
double excitation operators (pCCD), together with generalized single (S)
excitation operators. We compare its performance in both efficiency of
implementation and accuracy with that of the generalized UCC ansatz employing
the full generalized SD excitation operators (UCCGSD), as well as with the
standard ansatz employing only SD excitations (UCCSD). -UpCCGSD is found to
show the best scaling for quantum computing applications, requiring a circuit
depth of , compared with for UCCGSD and
for UCCSD where is the number of spin
orbitals and is the number of electrons. We analyzed the accuracy of
these three ans\"atze by making classical benchmark calculations on the ground
state and the first excited state of H (STO-3G, 6-31G), HO (STO-3G),
and N (STO-3G), making additional comparisons to conventional coupled
cluster methods. The results for ground states show that -UpCCGSD offers a
good tradeoff between accuracy and cost, achieving chemical accuracy for lower
cost of implementation on quantum computers than both UCCGSD and UCCSD. Excited
states are calculated with an orthogonally constrained variational quantum
eigensolver approach. This is seen to generally yield less accurate energies
than for the corresponding ground states. We demonstrate that using a
specialized multi-determinantal reference state constructed from classical
linear response calculations allows these excited state energetics to be
improved
QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion Quantum Monte Carlo
We review recent advances in the capabilities of the open source ab initio
Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for
greater efficiency and reproducibility. The auxiliary field QMC (AFQMC)
implementation has been greatly expanded to include k-point symmetries,
tensor-hypercontraction, and accelerated graphical processing unit (GPU)
support. These scaling and memory reductions greatly increase the number of
orbitals that can practically be included in AFQMC calculations, increasing
accuracy. Advances in real space methods include techniques for accurate
computation of band gaps and for systematically improving the nodal surface of
ground state wavefunctions. Results of these calculations can be used to
validate application of more approximate electronic structure methods including
GW and density functional based techniques. To provide an improved foundation
for these calculations we utilize a new set of correlation-consistent effective
core potentials (pseudopotentials) that are more accurate than previous sets;
these can also be applied in quantum-chemical and other many-body applications,
not only QMC. These advances increase the efficiency, accuracy, and range of
properties that can be studied in both molecules and materials with QMC and
QMCPACK
Photoionization of few electron systems with a hybrid Coupled Channels approach
We present the hybrid anti-symmetrized coupled channels method for the
calculation of fully differential photo-electron spectra of multi-electron
atoms and small molecules interacting with strong laser fields. The method
unites quantum chemical few-body electronic structure with strong-field
dynamics by solving the time dependent Schr\"odinger equation in a fully
anti-symmetrized basis composed of multi-electron states from quantum chemistry
and a one-electron numerical basis. Photoelectron spectra are obtained via the
time dependent surface flux (tSURFF) method. Performance and accuracy of the
approach are demonstrated for spectra from the helium and berryllium atoms and
the hydrogen molecule in linearly polarized laser fields at wavelength from 21
nm to 400 nm. At long wavelengths, helium and the hydrogen molecule at
equilibrium inter-nuclear distance can be approximated as single channel
systems whereas beryllium needs a multi-channel description
Increasing the representation accuracy of quantum simulations of chemistry without extra quantum resources
Proposals for near-term experiments in quantum chemistry on quantum computers
leverage the ability to target a subset of degrees of freedom containing the
essential quantum behavior, sometimes called the active space. This
approximation allows one to treat more difficult problems using fewer qubits
and lower gate depths than would otherwise be possible. However, while this
approximation captures many important qualitative features, it may leave the
results wanting in terms of absolute accuracy (basis error) of the
representation. In traditional approaches, increasing this accuracy requires
increasing the number of qubits and an appropriate increase in circuit depth as
well. Here we introduce a technique requiring no additional qubits or circuit
depth that is able to remove much of this approximation in favor of additional
measurements. The technique is constructed and analyzed theoretically, and some
numerical proof of concept calculations are shown. As an example, we show how
to achieve the accuracy of a 20 qubit representation using only 4 qubits and a
modest number of additional measurements for a simple hydrogen molecule. We
close with an outlook on the impact this technique may have on both near-term
and fault-tolerant quantum simulations
Efficient calculation of electronic structure using O(N) density functional theory
We propose an efficient way to calculate the electronic structure of large
systems by combining a large-scale first-principles density functional theory
code, Conquest, and an efficient interior eigenproblem solver, the
Sakurai-Sugiura method. The electronic Hamiltonian and charge density of large
systems are obtained by \conquest and the eigenstates of the Hamiltonians are
then obtained by the Sakurai-Sugiura method. Applications to a hydrated DNA
system, and adsorbed P2 molecules and Ge hut clusters on large Si substrates
demonstrate the applicability of this combination on systems with 10,000+ atoms
with high accuracy and efficiency.Comment: Submitted to J. Chem. Theor. Compu
Tensor Numerical Methods in Quantum Chemistry: from Hartree-Fock Energy to Excited States
We resume the recent successes of the grid-based tensor numerical methods and
discuss their prospects in real-space electronic structure calculations. These
methods, based on the low-rank representation of the multidimensional functions
and integral operators, led to entirely grid-based tensor-structured 3D
Hartree-Fock eigenvalue solver. It benefits from tensor calculation of the core
Hamiltonian and two-electron integrals (TEI) in complexity using
the rank-structured approximation of basis functions, electron densities and
convolution integral operators all represented on 3D
Cartesian grids. The algorithm for calculating TEI tensor in a form of the
Cholesky decomposition is based on multiple factorizations using algebraic 1D
``density fitting`` scheme. The basis functions are not restricted to separable
Gaussians, since the analytical integration is substituted by high-precision
tensor-structured numerical quadratures. The tensor approaches to
post-Hartree-Fock calculations for the MP2 energy correction and for the
Bethe-Salpeter excited states, based on using low-rank factorizations and the
reduced basis method, were recently introduced. Another direction is related to
the recent attempts to develop a tensor-based Hartree-Fock numerical scheme for
finite lattice-structured systems, where one of the numerical challenges is the
summation of electrostatic potentials of a large number of nuclei. The 3D
grid-based tensor method for calculation of a potential sum on a lattice manifests the linear in computational work, ,
instead of the usual scaling by the Ewald-type approaches
Extensions of the siesta dft code for simulation of molecules
We describe extensions to the siesta density functional theory (dft) code
[30], for the simulation of isolated molecules and their absorption spectra.
The extensions allow for: - Use of a multi-grid solver for the Poisson equation
on a finite dft mesh. Non-periodic, Dirichlet boundary conditions are computed
by expansion of the electric multipoles over spherical harmonics. - Truncation
of a molecular system by the method of design atom pseudo- potentials of Xiao
and Zhang[32]. - Electrostatic potential fitting to determine effective atomic
charges. - Derivation of electronic absorption transition energies and
oscillator stren- gths from the raw spectra produced by a recently described,
order O(N3), time-dependent dft code[21]. The code is furthermore integrated
within siesta as a post-processing option
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