208 citations · 569 across the 23 of their papers we have counts for
24 papers
Spectroscopic photorelaxation signatures in pyrazine from nonadiabatic dynamics simulations with coupled cluster theory
Sara Angelico, Eirik F. Kjønstad, Yi-Ping Chang +3
Despite extensive theoretical and experimental efforts, the mechanisms underlying the ultrafast relaxation of pyrazine after photoexcitation remain challenging to disentangle. Rece…
Can phaseless auxiliary-field quantum Monte Carlo with broken symmetry trials describe iron-sulfur clusters?
Eirik F. Kjønstad, Huanchen Zhai, James Shee +2
Phaseless auxiliary-field quantum Monte Carlo (AFQMC) has in several cases been found to perform well on strongly correlated systems. Here, we benchmark the method for three iron-s…
eT 2.0: An efficient open-source molecular electronic structure program
Sarai Dery Folkestad, Eirik F. Kjønstad, Alexander C. Paul +34
The eT program is an open-source electronic structure program with emphasis on performance and modularity. As its name suggests, the program features extensive coupled cluster capa…
Systematic improvement of trial states in phaseless auxiliary-field quantum Monte Carlo
Eirik F. Kjønstad, Yann Damour, Sandeep Sharma +1
We extend the use of coupled cluster (CC) trial states in the phaseless auxiliary-field quantum Monte Carlo (AFQMC) method beyond single and double excitations to include both trip…
Tensor Network Loop Cluster Expansions for Quantum Many-Body Problems
Johnnie Gray, Gunhee Park, Glen Evenbly +3
We analyze the tensor network loop cluster expansion, introduced in [G. Park, J. Gray, and G. K.-L. Chan, Phys. Rev. B 112, 174310 (2025)] as a systematic correction to belief prop…
Similarity Constrained CC2 for Efficient Coupled Cluster Nonadiabatic Dynamics
Leo Stoll, Sara Angelico, Eirik F. Kjønstad +1
Despite their high accuracy, standard coupled cluster models cannot be used for nonadiabatic molecular dynamics simulations because they yield unphysical complex excitation energie…
Generalized coupled cluster theory for ground and excited state intersections
Federico Rossi, Eirik F. Kjønstad, Sara Angelico +1
Coupled cluster theory in the standard formulation is unable to correctly describe conical intersections among states of the same symmetry. This limitation has restricted the pract…
Determining minimum energy conical intersections by enveloping the seam: exploring ground and excited state intersections in coupled cluster theory
Sara Angelico, Eirik F. Kjønstad, Henrik Koch
Minimum energy conical intersections can be used to rationalize photochemical processes. In this Letter, we examine an algorithm to locate these structures that does not require th…
Understanding failures in electronic structure methods arising from the geometric phase effect
Eirik F. Kjønstad, Henrik Koch
The geometric phase effect arises from the dependence on the nuclear coordinates in the electronic Hamiltonian, leading to sign changes of the electronic wave functions upon traver…
Fermionic tensor network contraction for arbitrary geometries
Yang Gao, Huanchen Zhai, Johnnie Gray +5
We describe our implementation of fermionic tensor network contraction on arbitrary lattices within both a globally ordered and locally ordered formalism. We provide a pedagogical…
Analytical evaluation of ground state gradients in quantum electrodynamics coupled cluster theory
Marcus Takvam Lexander, Sara Angelico, Eirik Fadum Kjønstad +1
Analytical gradients of potential energy surfaces play a central role in quantum chemistry, allowing for molecular geometry optimizations and molecular dynamics simulations. In str…
Unexpected hydrogen dissociation in thymine: predictions from a novel coupled cluster theory
Eirik F. Kjønstad, O. Jonathan Fajen, Alexander C. Paul +6
The fate of thymine upon excitation by ultraviolet radiation has been the subject of intense debate over the past three decades. Today, it is widely believed that its ultrafast exc…
Coupled cluster theory for nonadiabatic dynamics: nuclear gradients and nonadiabatic couplings in similarity constrained coupled cluster theory
Eirik F. Kjønstad, Sara Angelico, Henrik Koch
Coupled cluster theory is one of the most accurate electronic structure methods for predicting ground and excited state chemistry. However, the presence of numerical artifacts at e…
Prediction of Photodynamics of 200 nm Excited Cyclobutanone with Linear Response Electronic Structure and Ab Initio Multiple Spawning
Diptarka Hait, Dean Lahana, O. Jonathan Fajen +6
Simulations of photochemical reaction dynamics have been a challenge to the theoretical chemistry community for some time. In an effort to determine the predictive character of cur…
Non-adiabatic derivative coupling elements for the coupled cluster singles and doubles model
Eirik F. Kjønstad, Henrik Koch
We present an efficient implementation of analytical non-adiabatic derivative coupling elements for the coupled cluster singles and doubles model. The derivative coupling elements…
Efficient implementation of molecular CCSD gradients with Cholesky-decomposed electron repulsion integrals
Anna Kristina Schnack-Petersen, Henrik Koch, Sonia Coriani +1
We present an efficient implementation of ground and excited state CCSD gradients based on Cholesky-decomposed electron repulsion integrals. Cholesky decomposition, like density-fi…
A biorthonormal formalism for nonadiabatic coupled cluster dynamics
Eirik F. Kjønstad, Henrik Koch
In coupled cluster methods, the electronic states are biorthonormal in the sense that the left states are orthonormal to the right states. Here we present an extension of this form…
Multilevel CC2 and CCSD in reduced orbital spaces: electronic excitations in large molecular systems
Sarai D. Folkestad, Eirik F. Kjønstad, Linda Goletto +1
We present efficient implementations of the multilevel CC2 (MLCC2) and multilevel CCSD (MLCCSD) models. As the system size increases, MLCC2 and MLCCSD exhibit the scaling of the lo…
Coupled Cluster Theory for Molecular Polaritons: Changing Ground and Excited States
Tor S. Haugland, Enrico Ronca, Eirik F. Kjønstad +2
We present an ab initio correlated approach to study molecules that interact strongly with quantum fields in an optical cavity. Quantum electrodynamics coupled cluster theory provi…
Accelerated multimodel Newton-type algorithms for faster convergence of ground and excited state coupled cluster equations
Eirik F. Kjønstad, Sarai D. Folkestad, Henrik Koch
We introduce a multimodel approach to solve coupled cluster equations, employing a quasi Newton algorithm for the ground state and an Olsen algorithm for the excited states. In the…
eT 1.0: an open source electronic structure program with emphasis on coupled cluster and multilevel methods
Sarai D. Folkestad, Eirik F. Kjønstad, Rolf H. Myhre +14
The eT program is an open source electronic structure package with emphasis on coupled cluster and multilevel methods. It includes efficient spin adapted implementations of ground…
An efficient algorithm for Cholesky decomposition of electron repulsion integrals
Sarai D. Folkestad, Eirik F. Kjønstad, Henrik Koch
We present an algorithm where only the Cholesky basis is determined in the decomposition procedure. This allows for improved screening and a partitioned matrix decomposition scheme…
Resolving the notorious case of conical intersections for coupled cluster dynamics
Eirik F. Kjønstad, Henrik Koch
The motion of electrons and nuclei in photochemical events often involve conical intersections, degeneracies between electronic states. They serve as funnels for nuclear relaxation…
Crossing conditions in coupled cluster theory
Eirik F. Kjønstad, Rolf H. Myhre, Todd J. Martinez +1
We derive the crossing conditions at conical intersections between electronic states in coupled cluster theory, and show that if the coupled cluster Jacobian matrix is nondefective…