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ipie is a Python-based auxiliary-field quantum Monte Carlo (AFQMC) package that has undergone substantial improvements since its initial release [J. Chem. Theory Comput., 2022, 19(1): 109-121]. This paper outlines the improved modularity and new capabilities implemented in ipie. We highlight the ease of incorporating different trial and walker types and the seamless integration of ipie with external libraries. We enable distributed Hamiltonian simulations, allowing for multi-GPU simulations of large systems. This development enabled us to compute the interaction energy of a benzene dimer with 84 electrons and 1512 orbitals, which otherwise would not have fit on a single GPU. We also support GPU-accelerated multi-slater determinant trial wavefunctions [arXiv:2406.08314] to enable efficient and highly accurate simulations of large-scale systems. This allows for near-exact ground state energies of multi-reference clusters, [Cu$_2$O$_2$]$^{2+}$ and [Fe$_2$S$_2$(SCH$_3$)]$^{2-}$. We also describe implementations of free projection AFQMC, finite temperature AFQMC, AFQMC for electron-phonon systems, and automatic differentiation in AFQMC for calculating physical properties. These advancements position ipie as a leading platform for AFQMC research in quantum chemistry, facilitating more complex and ambitious computational method development and their applications.
Hedin's equations provide an elegant route to compute the exact one-body Green's function (or propagator) via the self-consistent iteration of a set of non-linear equations. Its first-order approximation, known as $GW$, corresponds to a resummation of ring diagrams and has shown to be extremely successful in physics and chemistry. Systematic improvement is possible, although challenging, via the introduction of vertex corrections. Considering anomalous propagators and an external pairing potential, we derive a new self-consistent set of closed equations equivalent to the famous Hedin equations but having as a first-order approximation the particle-particle (pp) $T$-matrix approximation where one performs a resummation of the ladder diagrams. This pp version of Hedin's equations offers a way to go systematically beyond the $T$-matrix approximation by accounting for low-order pp vertex corrections.
The Bethe–Salpeter equation (BSE) is the key equation in many-body perturbation theory based on Green's functions to access response properties. Within the GW approximation to the exchange-correlation kernel, the BSE has been successfully applied to several finite and infinite systems. However, it also shows some failures, such as underestimated triplet excitation energies, lack of double excitations, ground-state energy instabilities in the dissociation limit, etc. In this work, we study the performance of the BSE within the GW approximation as well as the T-matrix approximation for the excitation energies of the exactly solvable asymmetric Hubbard dimer. This model allows one to study various correlation regimes by varying the on-site Coulomb interaction U as well as the degree of the asymmetry of the system by varying the difference of potential Δv between the two sites. We show that, overall, the GW approximation gives more accurate excitation energies than GT over a wide range of U and Δv. However, the strongly correlated (i.e., large U) regime still remains a challenge.
We introduce a novel algorithm that leverages stochastic sampling techniques to compute the perturbative triples correction in the coupled-cluster (CC) framework. By combining elements of randomness and determinism, our algorithm achieves a favorable balance between accuracy and computational cost. The main advantage of this algorithm is that it allows for the calculation to be stopped at any time, providing an unbiased estimate, with a statistical error that goes to zero as the exact calculation is approached. We provide evidence that our semi-stochastic algorithm achieves substantial computational savings compared to traditional deterministic methods. Specifically, we demonstrate that a precision of 0.5 millihartree can be attained with only 10\% of the computational effort required by the full calculation. This work opens up new avenues for efficient and accurate computations, enabling investigations of complex molecular systems that were previously computationally prohibitive.
Sujets
AB-INITIO
Adiabatic connection
3470+e
X-ray spectroscopy
Perturbation theory
3115am
Relativistic corrections
Mécanique quantique relativiste
Pesticide
Molecular properties
Azide Anion
3115bw
Dirac equation
Electron electric dipole moment
Valence bond
Configuration interactions
Carbon Nanotubes
Molecular descriptors
Large systems
Relativistic quantum mechanics
QSAR
Argon
Fonction de Green
Numerical calculations
Time-dependent density-functional theory
AB-INITIO CALCULATION
Line formation
Parity violation
Chemical concepts
Coupled cluster calculations
BSM physics
3115vj
Atomic and molecular collisions
Basis set requirements
3115aj
Atomic and molecular structure and dynamics
Auto-énergie
Chimie quantique
3315Fm
Spin-orbit interactions
Atrazine-cations complexes
New physics
3115vn
Atom
AROMATIC-MOLECULES
Atrazine
Biodegradation
Acrolein
BIOMOLECULAR HOMOCHIRALITY
Hyperfine structure
Quantum Monte Carlo
BENZENE MOLECULE
Green's function
Time reversal violation
Configuration Interaction
Single-core optimization
CP violation
Corrélation électronique
Anderson mechanism
Atomic processes
3115ag
Aimantation
Diatomic molecules
Density functional theory
Ion
Electron correlation
Wave functions
Range separation
Configuration interaction
Petascale
Dipole
Atomic data
ALGORITHM
Dispersion coefficients
Quantum chemistry
Excited states
Analytic gradient
Relativistic quantum chemistry
Electron electric moment
Ground states
Ab initio calculation
Quantum Chemistry
Parallel speedup
A priori Localization
Pesticides Metabolites Clustering Molecular modeling Environmental fate Partial least squares
Abiotic degradation
Coupled cluster
CIPSI
Rydberg states
Approximation GW
Atoms
3115ae
Argile
Atomic charges chemical concepts maximum probability domain population
Atomic charges
A posteriori Localization
Xenon
États excités
Polarizabilities
Diffusion Monte Carlo