In this thesis numerical values are presented for the energies of the ground state of the He- H2 system, obtained with coupled-cluster (CC) methods at 223C 20,000 nuclear geometries. Approximately 68,000 ab initio calculations have been performed for the ground and first excited singlet state of He-H2. In these calculations, the H-H bond lengths range from 0.942 to 5.70 a0 (at 14 different values), the intermolecular separations R range from 0.25 to 20.0 a0 (usually at 74 different R values), and the angle &thetas; between r and R ranges from 0° to 90° in steps of 5 degrees. Characteristics of the potentials, the bound vibrational state, and the intersection between the states are investigated. The CCSD(T) method is known to yield unreliable results when internuclear separations are far from their equilibrium values, whereas the CR-CC(2,3) method has been shown to treat stretched bonds more accurately, the CR-CC(2,3) method was chosen for this work, because regions of the H2-He potential energy surface having stretched H-H bonds are desired with greater accuracy than in the earlier work. The calculations performed in this work also test the CR-CC(2,3) method for van der Waals molecules. The CR-CC(2,3) results in this work are compared with CCSD(T) results obtained in this work, and with CCSD(T) results in the scientific literature (where available). The parts of the potential surface corresponding to H2-He, H-H-He, H-He-H, HHe-H, H-HHe, HeH+-H - and the conical intersection between the ground and excited state have been calculated.
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