In an effort to obtain a complete description of electron correlation, highly accurate ab initio methods may be used in tandem with sufficiently large one-particle basis sets to target the exact correlation energy. The issue lies in the slow convergence of the methods and basis sets to reach the exact limit. In the first and second project of this work, we explore the use of focal point analysis to target the highly reliable CCSDTQ and CCSDT(Q) levels of theory, respectively. In the third project, we explore the accelerated convergence effect of explicit correlation by implementing explicitly correlated MP2. Additionally, automatic differentiation is employed with MP2-F12 to determine whether improved basis set convergence is achieved for nuclear gradients. Finally, in the last project, we compare high-level coupled cluster and the more approximate density functional theory to experimental VUV absorption spectra of various small molecules.
