First-principles studies of solvated molecules
This work is an investigation of the spectroscopy and thermodynamics of solvated molecules using cyanoacetylene in helium and methane in water as our two main case studies. For the former system, information on the spectra was obtained, while the latter was used as a prototype to study the hydrophobic effect. The most important elements needed in the study of these systems, namely, the intermolecular potential energy surfaces (PESs) were developed from first principles, i.e., without the use of any experimental data. In the case of the CH4-H2O system, the PESs were also used to calculate the cross second virial coefficient, useful in the natural-gas industry. The predictions of the spectra of the He-HCCCN dimer made in this work have now been confirmed by experiments. By extending the results for this dimer using a simple model, the reduction in the rotational constant of HCCCN upon solvation in a superfluid helium nanodroplet was explained and the value of this constant was predicted. The work on the hydrophobic effect involved molecular dynamics simulations of methane-water mixtures. In some of these calculations, a polarization model was used to go beyond the popularly-used pairwise additivity approximation of the interaction energies for many-body systems. The results from the molecular simulations increase our understanding of the nature of the hydrophobic effect by describing the influence of temperature on this phenomenon and also shedding light on the relation between the hydrophobic effect and the molecular properties of the solute molecules. The need to use the Helmholtz free energy and not simply one of its components (the entropic one as is often done) to explain the aggregation of apolar molecules in water was also emphasized.