# Cost-effective description of strong correlation: Efficient implementations of the perfect quadruples and perfect hextuples models.

@article{Lehtola2016CosteffectiveDO, title={Cost-effective description of strong correlation: Efficient implementations of the perfect quadruples and perfect hextuples models.}, author={Susi Lehtola and John A. Parkhill and Martin Head‐Gordon}, journal={The Journal of chemical physics}, year={2016}, volume={145 13}, pages={ 134110 } }

Novel implementations based on dense tensor storage are presented for the singlet-reference perfect quadruples (PQ) [J. A. Parkhill et al., J. Chem. Phys. 130, 084101 (2009)] and perfect hextuples (PH) [J. A. Parkhill and M. Head-Gordon, J. Chem. Phys. 133, 024103 (2010)] models. The methods are obtained as block decompositions of conventional coupled-cluster theory that are exact for four electrons in four orbitals (PQ) and six electrons in six orbitals (PH), but that can also be applied to…

## 16 Citations

### Orbital optimisation in the perfect pairing hierarchy: applications to full-valence calculations on linear polyacenes

- Physics
- 2018

ABSTRACT We describe the implementation of orbital optimisation for the models in the perfect pairing hierarchy. Orbital optimisation, which is generally necessary to obtain reliable results, is…

### Coupled-Cluster Valence-Bond Singles and Doubles for Strongly Correlated Systems: Block-Tensor Based Implementation and Application to Oligoacenes.

- ChemistryJournal of chemical theory and computation
- 2017

A block-tensor based implementation of coupled-cluster valence-bond singles and doubles (CCVB-SD) which is a simple modification to restricted CCSD that provides a qualitatively correct description of valence correlations even in strongly correlated systems.

### A Combined Selected Configuration Interaction and Many-Body Treatment of Static and Dynamical Correlation in Oligoacenes.

- Physics, ChemistryJournal of chemical theory and computation
- 2018

Large bases and reference relaxation lead to a significant reduction in the estimated radical character of the oligoacenes, with respect to previous valence-only treatments of correlation effects.

### Numerical and Theoretical Aspects of the DMRG-TCC Method Exemplified by the Nitrogen Dimer

- PhysicsJournal of chemical theory and computation
- 2019

The dependence of the ground-state energy error on thecomplete active space has been analyzed numerically in order to find an optimal split between the complete active space and external space by minimizing the density matrix renormalization group-tailored coupled-cluster error.

### Independent amplitude approximations in coupled cluster valence bond theory: Incorporation of 3-electron-pair correlation and application to spin frustration in the low-lying excited states of a ferredoxin-type tetrametallic iron-sulfur cluster.

- Chemistry, PhysicsThe Journal of chemical physics
- 2018

An extension of CCVB, termed CCVB+i3, is reported here that includes an independent (i) amplitude approximation to the 3-pair recouplings, and astonishingly, PP+i2 can also correctly separate systems that CCVB cannot.

### Automated Construction of Molecular Active Spaces from Atomic Valence Orbitals.

- ChemistryJournal of chemical theory and computation
- 2017

The atomic valence active space technique is introduced, a simple and well-defined automated technique for constructing active orbital spaces for use in multiconfiguration and multireference electronic structure calculations that makes MR calculations easier to execute, easier to reproduce by any user, and simplifies the determination of the appropriate size of the active space required for accurate results.

### Strong correlation in incremental full configuration interaction.

- ChemistryThe Journal of chemical physics
- 2017

The Perfect Pairing ansatz replaces the Hartree-Fock reference of the original iFCI method, and the resulting approach, PP-iFCI, is size consistent, size extensive, and systematically improvable with increasing order of incremental expansion.

### Coupled cluster valence bond theory for open-shell systems with application to very long range strong correlation in a polycarbene dimer.

- ChemistryThe Journal of chemical physics
- 2017

CCVB is successfully applied to the low-lying spin states of some model linear polycarbenes, systems that appear to be a hindrance to standard density functionals.

### Electronic Structure Tools for Transition Metal Complexes with Many Open Shells

- Physics
- 2018

In this thesis we present three different tools to model open shell transition metal electronic structure, commonly referred to as multi-reference (MR) or multiconfigurational electronic structure.…

### Cluster decomposition of full configuration interaction wave functions: A tool for chemical interpretation of systems with strong correlation.

- ChemistryThe Journal of chemical physics
- 2017

An implementation of the cluster decomposition analysis is presented and it is shown that connected octuple excitations still appear important in Cr2, suggesting that spin-restricted single-reference coupled-cluster approaches may not be tractable for some problems in transition metal chemistry.

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