Coupled Cluster Theory Thesis

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This is the first application of the new SSCC code, which uses an improved computational strategy for handling the semi-internal triexcited clusters.

Details of this new implementation of the SSCCSD(T) method are discussed.

Andre is a Swedish citizen, having studied and worked at KTH in Stockholm and Uppsala Universitet, Sweden.

Andre obtained his Ph D in Applied and Computational Mathematics from KTH in 2014.

In the present work, we want to introduce the mathematical framework of singular analysis and discuss a novel asymptotic parametrix construction for Hamiltonians of many-particle Coulomb systems.

This corresponds to the construction of an approximate inverse of a Hamiltonian operator with remainder given by a so-called Green operator.It is hoped that this method, where cluster amplitudes and spinorbitals are fully (bi)variational, may give good results for multireference-type systems. She obtained her MSc titled "Vibrational motion in molecules" from the Norwegian University of Science and Technology (NTNU) in 2014.After a few years working as a Project Engineer at Hydro in Årdal, she returned to theoretical chemistry, moving to Oslo and starting her Ph D fellowship at the BIVAQUM project in fall 2016.Simen has a broad background and experience from several scientific fields.He has a Ph D in Physics and Applied Mathematics from The Centre of Mathematics for Applications (CMA), University of Oslo, Norway, completed in 2009.The focus is on the asymptotic behaviour of ladder diagrams, which provide the dominant contribution to short-range correlation in coupled cluster theory.Furthermore, we discuss possible consequences of our asymptotic analysis with respect to adaptive wavelet approximation.The Green operator encodes essential asymptotic information and we present as our main result an explicit asymptotic formula for this operator.First applications to many-particle models in quantum chemistry are presented in order to demonstrate the feasibility of our approach.Both double zeta and double zeta plus polarization basis sets are employed and a few different choices of active space are considered. The SSCCSD(T) method provides an accurate description of the entire PES at low cost even for the bond breaking region, contrary to the results obtained with the perturbative single-reference CCSD(T) method or various limited configuration interaction approaches. Improved computational strategy for the state-selective coupled-cluster theory with semi-internal triexcited clusters: Potential energy surface of the HF molecule.