Publication of my PhD thesis

Finally, after quite some years of work I have successfully defended my PhD thesis with the title Development of a modular quantum-chemistry framework for the investigation of novel basis functions on 22nd May. An electronic version of the text has now been published online.

To make the topic of my work accessible both for chemists as well as mathematicians the thesis introduces the numerical modelling of chemical systems from a rather mathematical perspective. For example the first chapters are devoted to spectral theory as well as Ritz-Galerkin discretisation techniques and iterative diagonalisation algorithms. In the following chapters the main equations underlying quantum chemical modelling, i.e. the Schrödinger equation as well as the Hartree-Fock approximation, are presented stating known results about their mathematical properties. Afterwards the focus is shifted to numerical techniques for solving Hartree-Fock and further to developing a unified algorithmic approach for dealing with self-consistent field problems irrespective of the basis function type employed. This leads to a discussion of contraction-based methods, lazy matrices as well as the molsturm package. Last but not least convergence results for Coulomb-Sturmian-based quantum chemistry are presented.
For more details see the full abstract given below.

Published thesis versions

My thesis is made available in three forms. Firstly the officially published documents can be accessed using the DOI 10.11588/heidok.00024519. Please use this DOI as well as this bibtex entry for citations.

Secondly the LaTeX source code is made available on github. I will keep amending this repository as errata and typos are discovered. Note, that I deliberately do not publish this code as free software yet, since there are a few legal issues I want to clearify first. Please let me know, however, if you wish to use some of the LaTeX and CMake code and I am sure we can work something out.

Thirdly, from time to time I plan on making new "releases" out of the amended LaTeX code on github. The link https:/michael-herbst.com/publications/2018.05_phd_corrected.pdf will be used to point to the most recent version.

Full thesis abstract

State-of-the-art methods for the calculation of electronic structures of molecules predominantly use Gaussian basis functions. The algorithms employed inside existing code packages are consequently often highly optimised keeping only their numerical requirements in mind. For the investigation of novel approaches, utilising other basis functions, this is an obstacle, since requirements might differ. In contrast, this thesis develops the highly flexible program package molsturm, which is designed in order to facilitate rapid design, implementation and assessment of methods employing different basis function types. A key component of molsturm is a Hartree-Fock (HF) self-consistent field (SCF) scheme, which is suitable to be combined with any basis function type.

First the mathematical background of quantum mechanics as well as some numerical techniques are reviewed. Care is taken to emphasise the often overlooked subtleties when discretising an infinite-dimensional spectral problem in order to obtain a finite-dimensional eigenproblem. Common quantum-chemical methods such as full configuration interaction and HF are discussed providing insight into their mathematical properties. Different formulations of HF are contrasted and appropriate (SCF) solution schemes formulated.

Next discretisation approaches based on four different types of basis functions are compared both with respect to the computational challenges as well as their ability to describe the physical features of the wave function. Besides (1) Slater-type orbitals and (2) Gaussian-type orbitals, the discussion considers (3) finite elements, which are piecewise polynomials on a grid, as well as (4) Coulomb-Sturmians, which are the analytical solutions to a Schrödinger-like equation. A novel algorithmic approach based on matrix-vector contraction expressions is developed, which is able to adapt to the numerical requirements of all basis functions considered. It is shown that this ansatz not only allows to formulate SCF algorithms in a basis-function independent way, but furthermore improves the theoretically achievable computational scaling for finite-element-based discretisations as well as performance improvements for Coulomb-Sturmian-based discretisations. The adequacy of standard SCF algorithms with respect to a contraction-based setting is investigated and for the example of the optimal damping algorithm an approximate modification to achieve such a setting is presented.

With respect to recent trends in the development of modern computer hardware the potentials and drawbacks of contraction-based approaches are evaluated. One drawback, namely the typically more involved and harder-to-read code, is identified and a data structure named lazy matrix is introduced to overcome this. Lazy matrices are a generalisation of the usual matrix concept, suitable for encapsulating contraction expressions. Such objects still look like matrices from the user perspective, including the possibility to perform operations like matrix sums and products. As a result programming contraction-based algorithms becomes similarly convenient as working with normal matrices. An implementation of lazy matrices in the lazyten linear algebra library is developed in the course of the thesis, followed by an example demonstrating the applicability in the context of the HF problem.

Building on top of the aforementioned concepts the design of molsturm is outlined. It is shown how a combination of lazy matrices and a contraction-based SCF scheme separates the code describing the SCF procedure from the code dealing with the basis function type. It is discussed how this allows to add a new basis function type to molsturm by only making code changes in a single integral interface library. On top of that, we demonstrate by the means of examples how the readily scriptable interface of molsturm can be employed to implement and assess novel quantum-chemical methods or to combine the features of molsturm with existing third-party packages.

Finally, the thesis discusses an application of molsturm towards the investigation of the convergence properties of Coulomb-Sturmian-based quantum-chemical calculations. Results for the convergence of the ground-state energies at HF level are reported for atoms of the second and the third period of the periodic table. Particular emphasis is put on a discussion about the required maximal angular momentum quantum numbers in order to achieve convergence of the discretisation of the angular part of the wave function. Some modifications required for a treatment at correlated level are suggested, followed by a discussion of the effect of the Coulomb-Sturmian exponent. An algorithm for obtaining an optimal exponent is devised and some optimal exponents for the atoms of the second and the third period of the periodic table at HF level are given. Furthermore, the first results of a Coulomb-Sturmian-based excited states calculation based on the algebraic-diagrammatic construction scheme for the polarisation propagator are presented.

Acknowledgements

No doubt the research leading up to a PhD thesis cannot be successfully achieved without the help and support from many people, both scientifically as well as on a personal level. In the past years I have been very lucky to be surrounded by people, who created an environment for fruitful discussion. I am happy for everyone from related fields of science and technology, who broadened my own perspective by getting me in touch with new and unusual ideas and who helped me escape the box of my own narrow thinking. Many people should be named in this respect, most of them are already given credit in the acknowledgement section of my PhD thesis, so let me be rather brief here and simply say thanks!

Download thesis

To download the most recent version including corrections for all known errors and typos use this link. To cite the thesis please use this bibtex entry.

Thesis
Michael F. Herbst. Development of a modular quantum-chemistry framework for the investigation of novel basis functions.
PhD thesis, University of Heidelberg (2018).
[DOI 10.11588/heidok.00024519] [bib] [code] [errata] [further details]