Water is undoubtedly the most important liquid on our planet, as well as in our everyday life. Due to this fact, a huge amount of information has been gathered about its properties, from the isolated molecule/small groups of molecules to large bulks of the liquid, such as the oceans. In this presentation, I will focus on the atomic level structure of bulk liquid water: the reason is that the most essential characteristic, hydrogen bonding (H-bonding), has the most profound effect in this form -- and also, that there are still controversies concerning the basics of the microscopic structure.

I will briefly mention the so-called 'water anomalies', of which the best known is that water molecules naturally form a liquid phase under ambient conditions. Another well-known anomaly is the density maximum at 4 degrees C (which in itself has a major effect on life in natural waters). All the ca. 50 anomalies are due to the presence of the aforementioned peculiarity: hydrogen bonding.

I will elaborate on the problems with determining the most basic quantity, the first non-bonding O...H distance (the H-bonding distance). The intrinsic severe difficulties with neutron diffraction experiments, that should be sensitive to this distance, will be explained in some detail, along with efforts that have been made by the speaker over the past 25 years or so (see, e.g., [1]).

In the last section of the presentation, the possible exploitation of polarised neutron diffraction will be shown (see, e.g., [2]).

[1] L. Pusztai, Phys. Rev. B 60, 11851 (1999)

[2] L. Temleitner, A. Stunault, G. Cuello, L. Pusztai, Phys. Rev. B 92, 014201/1-6 (2015)