Molecular properties in the linear response regime and beyond with relativistic coupled-cluster

Abstract

This thesis mainly focuses on the development and implementation of new methods to study various types of response properties for molecules containing heavy elements. We implement static and frequency-dependent linear and quadratic response properties based upon relativistic coupled cluster wave function models. The validations are done by calculating various types of molecular properties such as frequency- (in)dependent (hyper)polarizability (purely electric), indirect spin-spin coupling constant (purely magnetic), and optical rotation (mixed electric-magnetic). Moreover, the current implementations also allow evaluation of the absorption cross-sections: the linear response code can evaluate the damped response functions, which can be used to calculate the one-photon absorption cross-sections, and the quadratic response code can evaluate the two-photon absorption cross-sections provided the wave functions of the target states exist. In addition, we also implement the equation-of-motion (EOM) coupled-cluster theory to evaluate the ionization potential, electron affinity, and excitation energy. The new EOM codes reproduce the results from the previous implementation in the program RELCCSD very well. All the codes are implemented on the new GPU-accelerated coupled cluster module Ex- aCorr in DIRAC. This module is designed for dealing with large systems and perform- ing efficiently coupled cluster calculations on modern supercomputer architectures. To further reduce the calculation cost, we implement the relativistic MP2 frozen natural orbitals (FNOs) to reduce the virtual orbital space in the correlated calculation. The pilot tests show with using FNOs, one can obtain reliable estimates for both energies and molecular properties with only half the size of the full spaces. Apart from the development work, this thesis also contains applications of the existing relativistic quantum chemistry models to obtain the highly accurate electronic structures and transition properties of molecules containing heavy elements. We discuss the importance of the evaluation of the relativistic effect and electron correlation on an equal footing and the corresponding impact on different topics such as molecular laser cooling and plasma physics.