The generation of highly accurate potential energy surfaces (PESs) for reactive processes represents a difficult challenge for modern electronic structure theory. Since chemical reactions often involve breaking and forming bonds or intermediate and transition state species, one must employ a methodology that provides a balanced and highly accurate description of varying levels of electronic degeneracy, but that is also practical enough to be applied to a wide range of chemical problems. Using small to medium sized systems, we examine the performance of two classes of coupled-cluster (CC) methods which are capable of accounting for the diverse electron correlation effects encountered in the majority of ground- and excited-state PES considerations. The first class of methods are the size-extensive completely renormalized (CR) CC approaches for ground-states and their equation of motion (EOM) CC extensions for excited-states, in which noniterative corrections due to higher-order excitations are added to the energies obtained with the standard CC and EOMCC approximations, such as CCSD (CC with singles and doubles) or EOMCCSD (EOMCC with singles and doubles), respectively. In particular, we focus on the left-eigenstate CR-CC(2,3) and CR-EOMCC(2,3) methods, in which a noniterative correction due to triple excitations is added to the CCSD or EOMCCSD energy, respectively, and, when necessary, a noniterative correction for quadruple excitations is also included via the CR-CC(2,3)+Q approach. A new variant of the CR-EOMCC(2,3) method, abbreviated as delta-CR-EOMCC(2,3), that can provide a size-intensive treatment of excitation energies, is discussed as well. The second class of methods considered here is the active-space variants of the electron-attached (EA) and ionized (IP) EOMCC theories, which utilize the idea of applying a linear electron-attaching or ionizing operator to the correlated, ground-state CC wave function of an N-electron closed-shell system in order to generate the ground and excited states of the related (N ± 1)-electron radical species of interest. These approaches use a physically motivated set of active orbitals to a priori select the dominant higher-order correlation effects to be included in the calculation, which significantly reduces the costs of the high-level EA-and IP-EOMCC approximations needed for obtaining accurate results for open-shell species without sacrificing accuracy. We have also developed a general extrapolation strategy for reducing the cost of generating PESs with correlated electronic structure methods using the concept of correlation energy scaling. Benchmark studies were performed to demonstrate typical accuracies for two types of PES extrapolation schemes, namely, the single-level PES extrapolation schemes, in which the essential quantity, the correlation energy scaling factor, is generated using only the quantum chemistry method of interest, and the dual-level PES extrapolation schemes, where lower-order approaches are used to estimate the correlation energy scaling factor corresponding to the method of interest. Unifying features of these PES extrapolation techniques are discussed, including the role of pivot geometries and base wave functions, and PES extrapolation to the complete basis set limit is examined as well. Finally, the most essential details of the new open-shell EOMCCSD and EA- and IP-EOMCC computer codes for the GAMESS software package, developed as part of this thesis research, are described.
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