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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
États excités
Aimantation
Anderson mechanism
Diatomic molecules
Adiabatic connection
Argon
Basis set requirements
Quantum Monte Carlo
Relativistic quantum chemistry
BENZENE MOLECULE
Carbon Nanotubes
Time reversal violation
Molecular properties
Analytic gradient
Biodegradation
Line formation
Density functional theory
CP violation
BSM physics
Ab initio calculation
Parallel speedup
Numerical calculations
Mécanique quantique relativiste
Corrélation électronique
A posteriori Localization
Pesticides Metabolites Clustering Molecular modeling Environmental fate Partial least squares
Atomic and molecular collisions
Atoms
Fonction de Green
Time-dependent density-functional theory
New physics
Quantum chemistry
Wave functions
Xenon
Ion
Chimie quantique
CIPSI
Quantum Chemistry
Chemical concepts
Atomic processes
3470+e
Spin-orbit interactions
Atomic charges
Azide Anion
Dispersion coefficients
Petascale
AB-INITIO CALCULATION
QSAR
Argile
AB-INITIO
Coupled cluster calculations
Atrazine-cations complexes
3115vj
Range separation
Large systems
Rydberg states
3315Fm
Excited states
Configuration interaction
Ground states
ALGORITHM
AROMATIC-MOLECULES
Polarizabilities
Atomic data
Dirac equation
Atrazine
Valence bond
Single-core optimization
Approximation GW
Electron electric dipole moment
3115ag
Configuration interactions
Configuration Interaction
Perturbation theory
Molecular descriptors
Pesticide
Dipole
Atomic and molecular structure and dynamics
Green's function
Abiotic degradation
A priori Localization
Coupled cluster
Relativistic corrections
3115ae
3115am
Auto-énergie
3115aj
Electron correlation
Atom
Diffusion Monte Carlo
Parity violation
Atomic charges chemical concepts maximum probability domain population
3115vn
BIOMOLECULAR HOMOCHIRALITY
Acrolein
X-ray spectroscopy
3115bw
Electron electric moment
Hyperfine structure
Relativistic quantum mechanics