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Subject: search.filters.subject.530Subject: search.filters.subject.540Institute: search.filters.UBSInstitut.Institut für Theoretische ChemieEnd date: 2022

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Now showing 1 - 5 of 5
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    Development of full configuration interaction quantum Monte Carlo methods for strongly correlated electron systems
    (2019) Dobrautz, Werner; Alavi, Ali (Prof. Dr.)
    Full Configuration Interaction Quantum Monte Carlo (FCIQMC) is a prominent method to calculate the exact solution of the Schrödinger equation in a finite antisymmetric basis and gives access to physical observables through an efficient stochastic sampling of the wavefunction that describes a quantum mechanical system. Although system-agnostic (black-box-like) and numerically exact, its effectiveness depends crucially on the compactness of the wavefunction: a property that gradually decreases as correlation effects become stronger. In this work, we present two -conceptually distinct- approaches to extend the applicability of FCIQMC towards larger and more strongly correlated systems. In the first part, we investigate a spin-adapted formulation of the FCIQMC algorithm, based on the Unitary Group Approach. Exploiting the inherent symmetries of the nonrelativistic molecular Hamiltonian results in a dramatic reduction of the effective Hilbert space size of the problem. The use of a spin-pure basis explicitly resolves the different spin-sectors, even when degenerate, and the absence of spin-contamination ensures the sampled wavefunction is an eigenfunction of the total spin operator. Moreover, targeting specific many-body states with conserved total spin allows an accurate description of chemical processes governed by the intricate interplay of them. We apply the above methodology to obtain results, not otherwise attainable with conventional approaches, for the spin-gap of the high-spin cobalt atom ground- and low-spin excited state and the electron affinity of scandium within chemical accuracy to experiment. Furthermore we establish the ordering of the scandium anion bound states, which has until now not been experimentally determined. In the second part, we investigate a methodology to explicitly incorporate electron correlation into the initial Ansatz of the ground state wavefunction. Such an Ansatz induces a compact description of the wavefunction, which ameliorates the sampling of the configuration space of a system with FCIQMC. Within this approach, we investigate the two-dimensional Hubbard model near half-filling in the intermediate interaction regime, where such an Ansatz can be exactly incorporated by a nonunitary similarity transformation of the Hamiltonian based on a Gutzwiller correlator. This transformation generates novel three-body interactions, tractable due to the stochastic nature of FCIQMC, and leads to a non-Hermitian effective Hamiltonian with extremely compact right eigenvectors. The latter fact allows application of FCIQMC to larger lattice sizes, well beyond the reach of the method applied to the original Hubbard Hamiltonian.
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    High-level rovibrational calculations on ketenimine
    (2021) Tschöpe, Martin; Schröder, Benjamin; Erfort, Sebastian; Rauhut, Guntram
    From an astrochemical point of view ketenimine (CH2CNH) is a complex organic molecule (COM) and therefore likely to be a building block for biologically relevant molecules. Since it has been detected in the star-forming region Sagittarius B2(N), it is of high relevance in this field. Although experimental data are available for certain bands, for some energy ranges such as above 1200 cm-1 reliable data virtually do not exist. In addition, high-level ab initio calculations are neither reported for ketenimine nor for one of its deuterated isotopologues. In this paper, we provide for the first time data from accurate quantum chemical calculations and a thorough analysis of the full rovibrational spectrum. Based on high-level potential energy surfaces obtained from explicitly correlated coupled-cluster calculations including up to 4-mode coupling terms, the (ro)vibrational spectrum of ketenimine has been studied in detail by variational calculations relying on rovibrational configuration interaction (RVCI) theory. Strong Fermi resonances were found for all isotopologues. Rovibrational infrared intensities have been obtained from dipole moment surfaces determined from the distinguishable cluster approximation. A comparison of the spectra of the CH2CNH molecule with experimental data validates our results, but also reveals new insight about the system, which shows very strong Coriolis coupling effects.
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    Development and application of embedded methods to strongly and weakly correlated systems
    (2022) Vitale, Eugenio; Alavi, Ali (Prof. Dr.)
    Coupled cluster (CC) theory is a popular and reliable tool in quantum chemistry due to its improvable hierarchy of methods able to rapidly converge to the full configuration interaction (FCI) limit in weakly correlated systems. Although it represents one of the most efficient single reference methods to treat many-body correlations with high accuracy and reliable outcomes, it yields qualitatively erroneous results when applied to strongly correlated systems. Within this thesis, the Distinguishable Cluster (DC) method (i.e., a small modification of CC amplitude equations able to qualitatively describe strongly correlated systems), is combined with FCI Quantum Monte Carlo (FCIQMC) in order to present a new tailored approach, the tailored DC (TDC), which is more accurate than the corresponding tailored CC and the pure DC. To demonstrate this, the method is first benchmarked with a variety of test cases and then further evaluated with computation of spin-state splittings in a few Fe(II) complexes. The systematic improvability of the TDC method is shown as the active space is increased. In the last part of the thesis, a further embedding scheme to treat strong correlation effects is evaluated. Specifically, the development and application of a screened Coulomb formalism is discussed. This simple approach inspired by Random Phase approximation (RPA) shows to be extremely efficient in the dissociation of one- and two-dimensional hydrogen systems.