Abstract
With this dissertation, we contribute to the study of molecular excitations by assessing and expanding a specific electronic structure method: quantum Monte Carlo (QMC). The excited states of molecules play a central role in activating many natural processes, such as human vision, and in the functioning of new technologies like solar panels. Their theoretical study is a very active field of research, but due to their complex nature, we still lack a standard procedure to analyze them.
On one side, in many photo-induced processes, the effects of the environment on the molecule become decisive and have to be taken into account, for example, by using mixed quantum/classical approaches. On the other, outside the ground state equilibrium, the potential energy surfaces to be described is very complex, and especially when multiple states interact, the available quantum chemistry methods often fail.
In this thesis, we work on improving the quantum mechanical description of excited states by focusing on QMC methods, a class of techniques for solving the Schrödinger equation in a stochastic manner. Lately, they are attracting increasing interest in the electronic structure community thanks to their favorable scaling with the number of electrons and the natural ease in parallelization. In the context of excited states, they are affirming as a valid alternative to other methods, especially for those complex cases where the cheapest options (such as time-dependent density functional theory) fail in the description.
Moreover, recent algorithmic advancements in the QMC community have made it possible to extend the description from small to medium and relatively large (about 100 non-hydrogen atoms) molecules and to compute accurate geometries in both ground and excited states.
Encouraged by this work, in this dissertation, we further investigate the use of QMC for the study of molecular excited states. In particular, in the first half of the manuscript (Chapters 3 and 4), we try to build robust protocols to compute vertical excitations. In the second (Chapters 5 and 6), we explore and try to address the problems connected to using QMC methods to describe excited-state relaxation.
On one side, in many photo-induced processes, the effects of the environment on the molecule become decisive and have to be taken into account, for example, by using mixed quantum/classical approaches. On the other, outside the ground state equilibrium, the potential energy surfaces to be described is very complex, and especially when multiple states interact, the available quantum chemistry methods often fail.
In this thesis, we work on improving the quantum mechanical description of excited states by focusing on QMC methods, a class of techniques for solving the Schrödinger equation in a stochastic manner. Lately, they are attracting increasing interest in the electronic structure community thanks to their favorable scaling with the number of electrons and the natural ease in parallelization. In the context of excited states, they are affirming as a valid alternative to other methods, especially for those complex cases where the cheapest options (such as time-dependent density functional theory) fail in the description.
Moreover, recent algorithmic advancements in the QMC community have made it possible to extend the description from small to medium and relatively large (about 100 non-hydrogen atoms) molecules and to compute accurate geometries in both ground and excited states.
Encouraged by this work, in this dissertation, we further investigate the use of QMC for the study of molecular excited states. In particular, in the first half of the manuscript (Chapters 3 and 4), we try to build robust protocols to compute vertical excitations. In the second (Chapters 5 and 6), we explore and try to address the problems connected to using QMC methods to describe excited-state relaxation.
| Original language | English |
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| Qualification | Doctor of Philosophy |
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| Supervisors/Advisors |
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| Award date | 16 Dec 2022 |
| Place of Publication | Enschede |
| Publisher | |
| Print ISBNs | 978-90-365-5494-7 |
| DOIs | |
| Publication status | Published - 16 Dec 2022 |