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Abstract

Computational modelling of molecules and materials with strongly correlated electrons hasbeen a long-standing challenge in the field of theoretical chemistry. While, conventional multireference wave function methods provide a robust way to account of strong electron correlation, the high cost associated with them prohibits their application even for moderately sized systems. This dissertation focuses on development and application of low-cost, chemically-guided multireference methods. The primary emphasis is on the localized active space self-consistent field (LASSCF) method. This method is designed for systems with strongly correlated orbitals that are localized in different regions of the system. LASSCF scales better than the conventional complete active space (CAS) method and gives qualitatively accurate results for various classes of compounds. To extend the applications of LASSCF beyond capturing qualitative behavior, we developed methods that account for further electron correlation. The LAS-PDFT (pair density functional theory) method is less susceptible to approximations of the wave function and shows better agreement to the corresponding CAS results even for systems for which LASSCF fails to do so. The LAS-State interaction method systematically restores entanglement between the fragments and provides better insights for practical applications. The LAS-Unitary coupled cluster (UCC) method (chapter 5) is designed for fault-tolerant quantum computers in order to leverage their power to do unitary operations in order to rebuild correlation between LAS fragments.Lastly, we discuss the hybrid multiconfiguration pair-density functional theory (HMC-PDFT) that provides a significant improvement over the conventional PDFT in calculating excitation energies for a wide range of systems. These methods greatly extend our reach to get qualitatively accurate wave functions and quantitatively accurate energies for a wide range of challenging systems at a significantly lower computational cost.

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