PhD Thesis defense: Development, implementation and applications of hybrid quantum chemistry models

PhD Program

José Aarón Rodríguez Jiménez
Conference Hall, Faculty of Chemistry UPV/EHU, Donostia / San Sebastián
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PhD Thesis defense: Development, implementation and applications of hybrid quantum chemistry models

Electron structure / Method Developments

Supervisors: David Casanova (DIPC, Ikerbasque) & Eduard Matito (DIPC, Ikerbasque)

The field of theoretical quantum chemistry aims to organize chemical laws, principles and rules by applying quantum mechanics to various challenges within the realm of chemistry. Central to this framework is the interconnection of the structure with the properties of molecular systems. This interconnection is used to provide theories and explanations for chemical phenomena, such as bonding, heats of formations and reactivity mechanisms among others.

This thesis is devoted to contribute to the enormous task of method development, oriented towards overcoming the limitations imposed by the large computational cost involved in the solutions to the many-body electronic Schrödinger equation. In this regard, achieving accurate or exact solutions, although theoretically feasible, in practice, the exact solution is prohibitively expensive to be computed for large systems.

Designing a method that delivers cheap and accurate approximate solutions must take into account the description of electronic correlation. This refers to the influence that the motion of one electron has on the motion of another in a many-electron system. Central to this description is the concept of electron correlation energy, defined as a measure for the energy difference between the exact solution and that captured by mean field models (e.g., Hartree Fock).

Scientific research works included in this thesis are focused to merge different strategies, suitable for simultaneously treat dynamic and nondynamic correlation, together in an unified framework. Initially, we explore the capabilities of wave function theory (WFT) methodologies with density functional theory (DFT), resulting in improved mingled WFT-DFT approximations. We further implement and test an alternative method, consisting in the use of a two-electron effective potential able to retrieve the dynamic correlation contribution to a complete active space self consistent field (CASSCF) wave function. Finally, we turn our attention to the applicability of a mixed scheme that incorporates spin-orbit coupling contributions in a perturbative way. Such calculations are based on the application of the spin-orbit mean-field approximation with non-relativistic wave functions. Obtained results demonstrate he suitability of the proposed methods for the accurate treatment of electronic correlation, and particularly for the description of open-shell systems.