Accurate computation of molecular properties from novel applications of quantum mechanical wavefunction method

High-accuracy quantum mechanical (QM) wavefunction methods have been applied to compute molecular properties of weakly-bound clusters. This work focuses on both extending the applicability of robust theoretical methods to larger systems and also determining the inherent accuracy of ab initio methods when compared to experimentally measured properties. Described here is the development of an efficient many-body approach that offers the ability to reduce both the time and the computational resources normally required to compute these properties reliably. The N-body:Many-body QM:QM technique has been extended to compute harmonic vibrational frequencies of clusters. Applying this methodology to small hydrogen-bonded clusters demonstrates that this approach yields both optimized geometries and harmonic vibrational frequencies in excellent agreement with the "gold standard" of correlated wavefunction methods, the CCSD(T) method, but with much greater efficiency. In addition, this work includes careful calibration studies to examine the basis set convergence of harmonic frequencies to determine which basis sets can be employed to obtain ab initio frequencies lying near the complete basis set (CBS) limit. These benchmark values are used to calibrate more efficient methods including ab initio methods, various density functional approximations and water potentials. Lastly, anharmonic vibrational frequencies and dissociation energies have been computed for small hydrogen-bonded dimers, allowing for a direct comparison to experiment.