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Title: Approximate variational coupled cluster theories
Author: Robinson, James
ISNI:       0000 0004 2734 1382
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2012
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This thesis presents a new family of single-determinantal ab initio electronic structure models ideal for the black-box computational investigation of molecular phenomena. These theories mimic Variational Coupled Cluster and achieve an approximately fulfilled variational upper bound property on the exact ground-state Schrodinger energy eigenvalue, while not exceeding the limiting computational complexity of the well-known CCSD method, and without sacrificing any other advantageous methodological property. In particular, these Approximate Vari- ational Coupled Cluster Theories are formulated through the minimization of a rigorously extensive and orbitally-invariant functional that treats certain limiting systems exactly. Unlike CCSD and related methods, for which it is extremely problematic, these theories are highly robust to the breakdown of the Hartree- Fock approximation that occurs when the single Slater determinant of minimal energy becomes an inadequate qualitative description of the true electronic wavefunction. Furthermore, presented results suggest that when the essential physics of strong non-dynamic correlation is captured by a near-variational ansatz, remaining dynamic correlation effects may be legitimately included perturbatively, with implications for the design of future electronic structure models because the failure of methods such as CCSD(T) to describe the dissociation of multiple bonds may be ascribed to the inadequate non-variational description of the electronic structure at the CCSD level, and not to a breakdown of perturbation theory; Optimized-Orbital Quasi-Variational Coupled Cluster Doubles with a perturbative treatment of triple excitations (OQVCCD(T)) is capable of predicting a physically correct and quantitatively accurate potential energy curve for diatomic nitrogen, N2, which has not been achieved, at the time of writing, by any other practical (O(o2v4)-iterative O(o3v4)-non-iterative) method based on Restricted Hartree-Fock theory. The method is demonstrated to be additionally suitable for the black-box description of singlet multiradicals through application to model hydrogen chains.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QD Chemistry