Date of Award


Document Type

Open Access Dissertation


Chemistry and Biochemistry


College of Arts and Sciences

First Advisor

Vitaly Rassolov


Incorporation of electron correlation to improve the accuracy of computations remains a driving force in quantum chemical method development. Traditionally, electron correlation effects are divided into static and dynamic correlation parts, and the inclusion of both results in methods that are impractical for large chemical systems. The goal of this doctoral research is to develop a method that efficiently accounts for both components of electron correlation in a separate but balanced manner. The approach focuses on combining a geminal method, called the antisymmetrized product of singlet-type, strongly orthogonal geminals (SSG), with dynamic correlation by either density functionals or a recently developed, linear, two particle, correlation operator.

The SSG method is a quantum chemical method that groups all electrons in a chemical system into pair functions called geminals. Within geminals, electrons can adapt multiple electronic configurations which allows the method to incorporate most static correlation. However, between geminals, the electron-electron repulsion is mean-field and a strong orthogonality constraint forbids intergeminal electron excitation. Combining SSG with electron correlation from a density functional, implemented in SSG(DFT), is shown to improve optimized properties of main group diatomic molecules at small basis sets. However, overcorrelation due to double counting of dynamic correlation is observed at larger basis sets. The SSG( ˆ C) method improves the situation by describing dynamic correlation while negating the double counting error inherent in SSG(DFT). In addition, the SSpG method is developed to relax the strong orthogonality constraint and allow intergeminal electronic excitation. The method provides a small portion of dynamic correlation energy unaccounted for in the aforementioned methods. The SSpG method is compared to non-orthogonal geminal methods and is shown to be a viable method to relax the constraint. Performance of all methods is analyzed from applications to representative sets of atoms and small molecules and compared to other electronic structure methods as benchmarks.


© 2015, Brett Allen Cagg

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