6,981 research outputs found
Approximate Green's Function Coupled Cluster Method Employing Effective Dimension Reduction
The Green's function coupled cluster (GFCC) method is a powerful many-body
tool for computing the electronic structure of molecular and periodic systems,
especially when electrons of the system are strongly correlated. However, for
the GFCC to be routinely used in the electronic structure calculations, robust
numerical techniques and approximations must be employed to reduce its high
computational overhead. In our recent studies, we demonstrated that the GFCC
equations can be solved directly in the frequency domain using iterative linear
solvers, which can be easily distributed in a massively parallel environment.
In the present work, we demonstrate a successful application of
model-order-reduction (MOR) techniques in the GFCC framework. Briefly, for a
frequency regime which requires high resolution spectral function, instead of
solving GFCC linear equation of full dimension for every single frequency
point, an efficiently-solvable linear system model of a reduced dimension may
be built upon projecting the original GFCC linear system onto a subspace. From
this reduced order model is obtained a reasonable approximation to the full
dimensional GFCC linear equations in both interpolative and extrapolative
spectral regions. Here, we show that the subspace can be properly constructed
in an iterative manner from the auxiliary vectors of the GFCC linear equations
at some selected frequencies within the spectral region of interest. During the
iterations, the quality of the subspace and the linear system model can be
systematically improved. The method is tested in terms of the efficiency and
accuracy of computing spectral functions for some typical molecular systems
such as carbon monoxide, 1,3-butadiene, benzene, and adenine. As a byproduct,
the obtained reduced order model may provide a high quality initial guess which
improves the convergence rate for the existing iterative linear solver.Comment: 29 pages, 8 figure
Approximate Exponential Integrators for Time-Dependent Equation-of-Motion Coupled Cluster Theory
With growing demand for time-domain simulations of correlated many-body
systems, the development of efficient and stable integration schemes for the
time-dependent Schr\"odinger equation is of keen interest in modern electronic
structure theory. In the present work, we present two novel approaches for the
formation of the quantum propagator for time-dependent equation-of-motion
coupled cluster theory (TD-EOM-CC) based on the Chebyshev and Arnoldi
expansions of the complex, non-hermitian matrix exponential, respectively. The
proposed algorithms are compared with the short-iterative Lanczos method of
Cooper, et al [J. Phys. Chem. A. 2021 125, 5438-5447], the fourth-order
Runge-Kutta method (RK4), and exact dynamics for a set of small but challenging
test problems. For each of the cases studied, both of the proposed integration
schemes demonstrate superior accuracy and efficiency relative to the reference
simulations.Comment: 28 pages, 4 figure
A Parallel, Distributed Memory Implementation of the Adaptive Sampling Configuration Interaction Method
Many-body simulations of quantum systems is an active field of research that
involves many different methods targeting various computing platforms. Many
methods commonly employed, particularly coupled cluster methods, have been
adapted to leverage the latest advances in modern high-performance
computing.Selected configuration interaction (sCI) methods have seen extensive
usage and development in recent years. However development of sCI methods
targeting massively parallel resources has been explored only in a few research
works. In this work, we present a parallel, distributed memory implementation
of the adaptive sampling configuration interaction approach (ASCI) for sCI. In
particular, we will address key concerns pertaining to the parallelization of
the determinant search and selection, Hamiltonian formation, and the
variational eigenvalue calculation for the ASCI method. Load balancing in the
search step is achieved through the application of memory-efficient determinant
constraints originally developed for the ASCI-PT2 method. Presented benchmarks
demonstrate parallel efficiency exceeding 95\% for the variational ASCI
calculation of Cr (24e,30o) with , and variational
determinants up to 16,384 CPUs. To the best of the authors' knowledge, this is
the largest variational ASCI calculation to date.Comment: 32 pages, 4 figure
On the Efficient Evaluation of the Exchange Correlation Potential on Graphics Processing Unit Clusters
The predominance of Kohn-Sham density functional theory (KS-DFT) for the
theoretical treatment of large experimentally relevant systems in molecular
chemistry and materials science relies primarily on the existence of efficient
software implementations which are capable of leveraging the latest advances in
modern high performance computing (HPC). With recent trends in HPC leading
towards in increasing reliance on heterogeneous accelerator based architectures
such as graphics processing units (GPU), existing code bases must embrace these
architectural advances to maintain the high-levels of performance which have
come to be expected for these methods. In this work, we purpose a three-level
parallelism scheme for the distributed numerical integration of the
exchange-correlation (XC) potential in the Gaussian basis set discretization of
the Kohn-Sham equations on large computing clusters consisting of multiple GPUs
per compute node. In addition, we purpose and demonstrate the efficacy of the
use of batched kernels, including batched level-3 BLAS operations, in achieving
high-levels of performance on the GPU. We demonstrate the performance and
scalability of the implementation of the purposed method in the NWChemEx
software package by comparing to the existing scalable CPU XC integration in
NWChem.Comment: 26 pages, 9 figure
Real-Time Krylov Theory for Quantum Computing Algorithms
Quantum computers provide new avenues to access ground and excited state
properties of systems otherwise difficult to simulate on classical hardware.
New approaches using subspaces generated by real-time evolution have shown
efficiency in extracting eigenstate information, but the full capabilities of
such approaches are still not understood. In recent work, we developed the
variational quantum phase estimation (VQPE) method, a compact and efficient
real-time algorithm to extract eigenvalues on quantum hardware. Here we build
on that work by theoretically and numerically exploring a generalized Krylov
scheme where the Krylov subspace is constructed through a parametrized
real-time evolution, which applies to the VQPE algorithm as well as others. We
establish an error bound that justifies the fast convergence of our spectral
approximation. We also derive how the overlap with high energy eigenstates
becomes suppressed from real-time subspace diagonalization and we visualize the
process that shows the signature phase cancellations at specific eigenenergies.
We investigate various algorithm implementations and consider performance when
stochasticity is added to the target Hamiltonian in the form of spectral
statistics. To demonstrate the practicality of such real-time evolution, we
discuss its application to fundamental problems in quantum computation such as
electronic structure predictions for strongly correlated systems
Accurate Assignments of Excited-State Resonance Raman Spectra: A Benchmark Study Combining Experiment and Theory
This is an unofficial translation of an article that appeared in an ACS publication. ACS has not endorsed the content of this translation or the context of its use.Femtosecond stimulated Raman scattering (FSRS) probes the structural dynamics of molecules in electronically excited states by following the evolution of the vibrational spectrum. Interpreting the dynamics requires accurate assignments to connect the vibrational bands with specific nuclear motions of an excited molecule. However, the assignment of FSRS signals is often complicated by mode-specific resonance enhancement effects that are difficult to calculate for molecules in electronically excited states. We present benchmark results for a series of eight aryl-substituted thiophene derivatives to show that calculated off-resonance Raman spectra can be used to assign experimental bands on the basis of a comparison of structurally similar compounds and careful consideration of the resonance condition. Importantly, we show that direct comparison with the off-resonant calculations can lead to incorrect assignments of the experimental spectrum if the resonance condition is neglected. These results highlight the importance of resonance enhancement effects in assigning FSRS spectra
Distributed Memory, GPU Accelerated Fock Construction for Hybrid, Gaussian Basis Density Functional Theory
With the growing reliance of modern supercomputers on accelerator-based
architectures such a GPUs, the development and optimization of electronic
structure methods to exploit these massively parallel resources has become a
recent priority. While significant strides have been made in the development of
GPU accelerated, distributed memory algorithms for many-body (e.g.
coupled-cluster) and spectral single-body (e.g. planewave, real-space and
finite-element density functional theory [DFT]), the vast majority of
GPU-accelerated Gaussian atomic orbital methods have focused on shared memory
systems with only a handful of examples pursuing massive parallelism on
distributed memory GPU architectures. In the present work, we present a set of
distributed memory algorithms for the evaluation of the Coulomb and
exact-exchange matrices for hybrid Kohn-Sham DFT with Gaussian basis sets via
direct density-fitted (DF-J-Engine) and seminumerical (sn-K) methods,
respectively. The absolute performance and strong scalability of the developed
methods are demonstrated on systems ranging from a few hundred to over one
thousand atoms using up to 128 NVIDIA A100 GPUs on the Perlmutter
supercomputer.Comment: 45 pages, 9 figure
Estimating Eigenenergies from Quantum Dynamics: A Unified Noise-Resilient Measurement-Driven Approach
Ground state energy estimation in physics and chemistry is one of the most
promising applications of quantum computing. In this paper, we introduce a
novel measurement-driven approach that finds eigenenergies by collecting
real-time measurements and post-processing them using the machinery of dynamic
mode decomposition (DMD). We provide theoretical and numerical evidence that
our method converges rapidly even in the presence of noise and show that our
method is isomorphic to matrix pencil methods developed independently across
various scientific communities. Our DMD-based strategy can systematically
mitigate perturbative noise and stands out as a promising hybrid
quantum-classical eigensolver
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