Water can be described as the 'medium of Life'; it has tremendous importance in several fields of biology, geology and physics, for example. Therefore it is of crucial importance to describe its basic structure and properties accurately.
The apparent simplicity of water, and ice, originates of its composition of only two elements in a molecule and the interaction between the molecules is usually explained as strong hydrogen bonds, of electro-static nature. The phase diagrams of ice and water, however, already implies the complexity of water, leading to the diversity of anomalies in water.
Classical atomistic models often yield satisfactory results for the structural properties of liquid water in molecular simulations, yet either a parametrised description or a more sophisticated calculation is required to inspect the spectroscopic quantities. An ab initio calculation is required for a comprehensive and quantitative comparison with the experimental results.
Previous ab initio molecular dynamics simulations of liquid water have lead to somewhat an unsatisfactory state of agreement on the structure of water: Different calculations have resulted in different radial distribution functions, diffusion constants, and even in a different interpretation of path integral simulations. Possible origins of errors are the exchange-correlation functional employed, insufficient sampling, quantum or finite-size effects, inaccuracies in the numerical description of the electronic structure. For this reason it is also important to achieve as realistic and as many comparisons with the experimental probes as possible, and not rely eg. solely on structural data from neutron scattering experiments.
Sometimes, however, already the interpretation and analysis of the experimental results are also ambiguous. Therefore it is important to simulate also the experimental methods starting from the electronic structure and atomic configurations along the molecular dynamics trajectories. So far at least the Raman and infra-red vibrational spectroscopies, nuclear magnetic resonance (NMR), X-ray adsorption spectroscopies (XAS), optical spectroscopy have been used to model water.