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An automated method for systematic hierarchical fragmentation of large molecules is presented utilizing isodesmic reactions. It is shown that by taking appropriate linear combination of fragment energies, it is possible to obtain highly accurate total energies when compared to total energy of full molecule.

An extensive study on the harmonic frequencies of a large set of small polyatomic closed-shell molecules computed at both single level ab initio and composite approximations is presented. Using various combinations of basis sets, composite methods are capable of predicting single level ab initio harmonic frequencies, which suggest a computationally affordable means of obtaining highly accurate vibrational frequencies.

First principles NMR calculations of the nuclear magnetic shielding tensor were performed. It is shown that by utilizing the fragmentation approach molecular property for a large class of molecules allows construction of a database of molecular fragments that could be used in NMR characterization of molecules.

The isomers of hydroxyacetaldehyde were studied by both two and eighteen dimensional quantum diffusion Monte Carlo (DMC) calculations. A total of four unique minima, and the transition states connecting them were observed and the results obtained indicate that not all of the isomers can be isolated in the gas phase.

The elemental frequencies of vibration of first-row closed-shell diatomics are estimated using both the levels of the ab initio theory. The results indicate that accurate fundamental frequencies can be computed economically and effectively using a precise composite method.