7,893 research outputs found
Calculation of interatomic forces and optimization of molecular geometry with auxiliary-field quantum Monte Carlo
We propose an algorithm for accurate, systematic and scalable computation of
interatomic forces within the auxiliary-field Quantum Monte Carlo (AFQMC)
method. The algorithm relies on the Hellman-Fenyman theorem, and incorporates
Pulay corrections in the presence of atomic orbital basis sets. We benchmark
the method for small molecules by comparing the computed forces with the
derivatives of the AFQMC potential energy surface, and by direct comparison
with other quantum chemistry methods. We then perform geometry optimizations
using the steepest descent algorithm in larger molecules. With realistic basis
sets, we obtain equilibrium geometries in agreement, within statistical error
bars, with experimental values. The increase in computational cost for
computing forces in this approach is only a small prefactor over that of
calculating the total energy. This paves the way for a general and efficient
approach for geometry optimization and molecular dynamics within AFQMC.Comment: 5 pages, 4 figure
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
The Coupled Electron-Ion Monte Carlo Method
In these Lecture Notes we review the principles of the Coupled Electron-Ion
Monte Carlo methods and discuss some recent results on metallic hydrogen.Comment: 38 pages, 6 figures, Lecture notes for the International School of
Solid State Physics, 34th course: "Computer Simulation in Condensed Matter:
from Materials to Chemical Biology", 20 July-1 August 2005 Erice (Italy). To
appear in Lecture Notes in Physics (2006
From real materials to model Hamiltonians with density matrix downfolding
Due to advances in computer hardware and new algorithms, it is now possible
to perform highly accurate many-body simulations of realistic materials with
all their intrinsic complications. The success of these simulations leaves us
with a conundrum: how do we extract useful physical models and insight from
these simulations? In this article, we present a formal theory of
downfolding--extracting an effective Hamiltonian from first-principles
calculations. The theory maps the downfolding problem into fitting information
derived from wave functions sampled from a low-energy subspace of the full
Hilbert space. Since this fitting process most commonly uses reduced density
matrices, we term it density matrix downfolding (DMD).Comment: 24 pages, 12 figures; Huihuo Zheng and Hitesh J. Changlani
contributed equally to this wor
Algorithmic differentiation and the calculation of forces by quantum Monte Carlo
We describe an efficient algorithm to compute forces in quantum Monte Carlo
using adjoint algorithmic differentiation. This allows us to apply the space
warp coordinate transformation in differential form, and compute all the 3M
force components of a system with M atoms with a computational effort
comparable with the one to obtain the total energy. Few examples illustrating
the method for an electronic system containing several water molecules are
presented. With the present technique, the calculation of finite-temperature
thermodynamic properties of materials with quantum Monte Carlo will be feasible
in the near future.Comment: 32 pages, 4 figure, to appear in The Journal of Chemical Physic
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