Development and implementation of high accuracy coupled cluster methods for ground and excited state : applications to thermochemistry and spectroscopys
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Several methods for treating ground and excited states are presented. All of them are based on the coupled cluster theory which is acknowledged to provide an excellent alternative to the correlation problem. A new approach is developed and implemented for treating the quadruple excitations in coupled cluster theory. Quadruple excitation contributions are computed from a formula based on a non-Hermitian perturbation theory analogous to that used previously to justify the usual noniterative triples correction used in the coupled cluster method known as CCSD(T). This method is similar to CCSD(T) and is known as CCSDT(Q). The latter is thoroughly tested on a set of molecules and is also used to simplify the HEAT protocol for calculating thermochemical parameters. The total energies obtained with CCSDT(Q) agree favorably with the ones obtained with coupled cluster methods including single, double, triple, quadruple and even pentuple excitations. The enthalpies of formations calculated by combining CCSDT(Q) with the HEAT protocol are less than 1 kJ per mole of their best known values. Additionally, methods for evaluating ionization potentials with an approximate treatment of triple excitations are described. These are an extension of the existing EOMIPCCSD method and are based on the CCSDT-X (X=1a, 1b, 2, 3) and CC3 approximations in traditional coupled cluster theory. Appropriate to their role in studying radicals, they are here tested on the first two Σ states and first Π state of the N+ 2 , CO+, CN, and BO diatomic radicals. The calculations show a tendency for the CC3 variant to overestimate the bond lengths and to underestimate the vibrational frequencies, while the CCSDT-3 variant seems to be most reliable. It is also demonstrated that the accuracy of such methods is comparable to sophisticated traditional multireference approaches and full configuration interaction. Similar methods for studying excited states (EOMEE-CC) are used in combination with traditional coupled cluster methods and the vibronic coupling model to investigate the vertical excitation energy for the lowest valence π → π ∗ transition in cyclopentadiene (CP).