Higher excitations for an exponential multireference wavefunction Ansatz and single-reference based multireference coupled cluster Ansatz: Application to model systems H4, P4, and BeH2

2008 ◽  
Vol 128 (15) ◽  
pp. 154118 ◽  
Author(s):  
Michael Hanrath
Keyword(s):  
Model Systems ◽  
Coupled Cluster ◽  
Wavefunction Ansatz

Excitonic Coupled-cluster Theory: Part I, General Formalism

2017 ◽  
Author(s):  
Yuhong Liu ◽  
Anthony Dutoi
Keyword(s):  
Electron Correlation ◽  
Degrees Of Freedom ◽  
Model Systems ◽  
Coupled Cluster ◽  
Effective Hamiltonian ◽  
Coupled Cluster Theory ◽  
Cluster Theory ◽  
Companion Article ◽  
High Level ◽  
Fragment Interaction

<div> <div>A shortcoming of presently available fragment-based methods is that electron correlation (if included) is described at the level of individual electrons, resulting in many redundant evaluations of the electronic relaxations associated with any given fluctuation. A generalized variant of coupled-cluster (CC) theory is described, wherein the degrees of freedom are fluctuations of fragments between internally correlated states. The effects of intra-fragment correlation on the inter-fragment interaction is pre-computed and permanently folded into the effective Hamiltonian. This article provides a high-level description of the CC variant, establishing some useful notation, and it demonstrates the advantage of the proposed paradigm numerically on model systems. A companion article shows that the electronic Hamiltonian of real systems may always be cast in the form demanded. This framework opens a promising path to build finely tunable systematically improvable methods to capture precise properties of systems interacting with a large number of other systems. </div> </div>

Excitonic Coupled-cluster Theory: Part I, General Formalism

2017 ◽  
Author(s):  
Yuhong Liu ◽  
Anthony Dutoi
Keyword(s):  
Electron Correlation ◽  
Degrees Of Freedom ◽  
Model Systems ◽  
Coupled Cluster ◽  
Effective Hamiltonian ◽  
Coupled Cluster Theory ◽  
Cluster Theory ◽  
Companion Article ◽  
High Level ◽  
Fragment Interaction

<div> <div>A shortcoming of presently available fragment-based methods is that electron correlation (if included) is described at the level of individual electrons, resulting in many redundant evaluations of the electronic relaxations associated with any given fluctuation. A generalized variant of coupled-cluster (CC) theory is described, wherein the degrees of freedom are fluctuations of fragments between internally correlated states. The effects of intra-fragment correlation on the inter-fragment interaction is pre-computed and permanently folded into the effective Hamiltonian. This article provides a high-level description of the CC variant, establishing some useful notation, and it demonstrates the advantage of the proposed paradigm numerically on model systems. A companion article shows that the electronic Hamiltonian of real systems may always be cast in the form demanded. This framework opens a promising path to build finely tunable systematically improvable methods to capture precise properties of systems interacting with a large number of other systems. </div> </div>

High-order excitations in state-universal and state-specific multireference coupled cluster theories: Model systems

2006 ◽  
Vol 125 (15) ◽  
pp. 154113 ◽  
Author(s):  
Francesco A. Evangelista ◽  
Wesley D. Allen ◽  
Henry F. Schaefer
Keyword(s):  
High Order ◽  
Model Systems ◽  
Coupled Cluster

Triple excitations in state-specific multireference coupled cluster theory: Application of Mk-MRCCSDT and Mk-MRCCSDT-n methods to model systems

2008 ◽  
Vol 128 (12) ◽  
pp. 124104 ◽  
Author(s):  
Francesco A. Evangelista ◽  
Andrew C. Simmonett ◽  
Wesley D. Allen ◽  
Henry F. Schaefer ◽  
Jürgen Gauss
Keyword(s):  
Model Systems ◽  
Coupled Cluster ◽  
Coupled Cluster Theory ◽  
Cluster Theory ◽  
Theory Application

scholarly journals Complex Excited State Polarizabilities in the ADC/ISR Framework

2020 ◽  
Author(s):  
Maximilian Scheurer ◽  
Thomas Fransson ◽  
Patrick Norman ◽  
Andreas Dreuw ◽  
Dirk R. Rehn
Keyword(s):  
Excited States ◽  
Model Systems ◽  
Coupled Cluster ◽  
Complex Frequency ◽  
State Representation ◽  
Third Order ◽  
Dispersion Coefficients ◽  
Coupled Cluster Theory ◽  
Small Model ◽  
Complex Polarizability

<div><div><div><p>We present the derivation and implementation of complex, frequency-dependent polarizabilities for excited states using the algebraic-diagrammatic construction for the polarization propagator (ADC) and its intermediate state representation (ISR). Based on the complex polarizability we evaluate C<sub>6</sub> dispersion coefficients for excited states. The methodology is implemented up to third order in perturbation theory in the Python-driven adcc toolkit for the development and application of ADC methods. We exemplify the approach using small model systems and compare it to results from coupled-cluster theory and from experiments.</p></div></div></div>

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