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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.