Browsing NTNU Open by Author "Kjønstad, Eirik Fadum"
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An efficient algorithm for Cholesky decomposition of electron repulsion integrals
Folkestad, Sarai Dery; Kjønstad, Eirik Fadum; Koch, Henrik (Journal article; Peer reviewed, 2019)Approximating the electron repulsion integrals using inner projections is a well-established approach to reduce the computational demands of electronic structure calculations. Here, we present a two-step Cholesky decomposition ... -
An Orbital Invariant Similarity Constrained Coupled Cluster Model
Kjønstad, Eirik Fadum; Koch, Henrik (Journal article; Peer reviewed, 2019)We present a similarity constrained coupled cluster method able to describe conical intersections between two excited electronic states of the same symmetry. For a given pair of states, this singles and doubles method ... -
Crossing conditions in coupled cluster theory
Kjønstad, Eirik Fadum; Heilemann Myhre, Rolf; Martinez, Todd J.; Koch, Henrik (Journal article; Peer reviewed, 2017)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, two (three) independent conditions are ... -
Equation-of-motion coupled-cluster method with double electron-attaching operators: Theory, implementation, and benchmarks
Gulania, Sahil; Kjønstad, Eirik Fadum; Stanton, John F.; Koch, Henrik; Krylov, Anna I. (Journal article, 2021)We report a production-level implementation of the equation-of-motion (EOM) coupled-cluster (CC) method with double electron-attaching (DEA) EOM operators of 2p and 3p1h types, EOM-DEA-CCSD. This ansatz, suitable for ... -
Modeling Chemistry in QED Cavities using Coupled Cluster
Haugland, Tor Strømsem (Master thesis, 2019)I denne avhandlingen introduseres kvanteelektrodynamisk ''coupled cluster'' teori (QED-CC) som en ny og nøyaktig ab initio teori for å undersøke sterk kobling mellom lys og materie i optiske kaviteter. Ligningene for ... -
Multilevel CC2 and CCSD in Reduced Orbital Spaces: Electronic Excitations in Large Molecular Systems
Folkestad, Sarai Dery; Kjønstad, Eirik Fadum; Goletto, Linda; Koch, Henrik (Peer reviewed; Journal article, 2021)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 lower-level coupled cluster model. To treat ... -
Resolving the Notorious Case of Conical Intersections for Coupled Cluster Dynamics
Kjønstad, Eirik Fadum; Koch, Henrik (Journal article; Peer reviewed, 2017)The motion of electrons and nuclei in photochemical events often involves conical intersections, or degeneracies between electronic states. They serve as funnels in nuclear relaxation processes where the electrons and ... -
Spin adapted implementation of EOM-CCSD for triplet excited states: Probing intersystem crossings of acetylacetone at the carbon and oxygen K-edges
Faber, Rasmus; Kjønstad, Eirik Fadum; Koch, Henrik; Coriani, Sonia (Journal article; Peer reviewed, 2019)We present an equation of motion coupled cluster singles and doubles approach for computing transient absorption spectra from a triplet excited state. The implementation determines the left and right excitation vectors by ... -
Symmetric Coupled Cluster Theory
Kjønstad, Eirik Fadum (Master thesis, 2016)We introduce a novel coupled cluster formulation for the ground state, symmetric coupled cluster (SCC) theory, and an extension to excited states, molecular properties, and transition elements called symmetric equation of ...