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Rebecca Tomann

To predict physical properties of molecules computationally, ab initio methods such as coupled cluster and many body perturbation theory have been developed. Due to their high technical requirements (memory, disk space, CPU time), lowering the computational cost while maintaining the same level of accuracy of ab initio methods has been of great interest. One such approach towards this goal is an ab initio composite approach, which approximates a higher level of theory at a fraction of the computational cost through a combination of lower level ab initio calculations. The correlation consistent Composite Approach (ccCA) is one of the more accurate composite methodologies used to calculate thermochemical properties within chemical accuracy (1 kcal/mol) in comparison to experimental data. To determine the efficacy of coupled cluster and perturbation theory variants within the ccCA framework, the enthalpy of formation was predicted for a molecule set of 148 closed-shell, radical, and open-shell species consisting of first-row and second-row main group atoms. All variants were compared against the standard ccCA methodology for performance.