16
1 A Historical Review of the Structures of Water and Ice
g
(2) Potential
energy minimum
(1) Timeaveraged
m
Fig. 1.12 The rotating-pendulum model that describes the difference between (1) the time-averaged
position and (2) the potential-energy-minimum point. An inverted position is more likely at averaging as the pendulum spends there more time. However, it would never be observed if one searches
the equilibrium by the minimization of potential energy (see the text for analogy with the molecular
motion)
Three main problems prevent the understanding of the local structure of water by
RDF analysis. First, the result is significantly determined by the initial assumptions,
which are not yet well defined. As a result, we get significantly different results for
RDF, depending on the molecular potential that we use. Second, the distribution function reflects the time- and space-averaged configurations of short-lived local molecular environments. This fact makes the analysis of the potential-energy-minimum
F-structure controversial. For example, the averaged position for rotating pendulum
(see Fig. 1.12) corresponds to the upper point, because it spends more time there
than at any other point, while the potential-energy-minimum position is obviously
at the bottom. This analogy shows the difference between the diffusion-averaged
and potential-energy-minimum structures. Third, although the concept of interstitial
molecules qualitatively explains the denser configuration of water in comparison with
ice, it was not confirmed by high-pressure experiments [46]. The expected increase
of the concentration of interstitial molecules at higher pressure in this model has not
been observed experimentally.
Therefore, we cannot learn more about water structure using the RDF only. The
RDF confirms the roughly tetrahedral arrangement of molecules, but does not provide
the angular distribution among neighboring molecules, nor does it reflect the distribution around the local atom within short periods of time. Thus, the RDF analysis
highlights the D-structure but says nothing about the I- or V- structures [3].
1 A Historical Review of the Structures of Water and Ice
g
(2) Potential
energy minimum
(1) Timeaveraged
m
Fig. 1.12 The rotating-pendulum model that describes the difference between (1) the time-averaged
position and (2) the potential-energy-minimum point. An inverted position is more likely at averaging as the pendulum spends there more time. However, it would never be observed if one searches
the equilibrium by the minimization of potential energy (see the text for analogy with the molecular
motion)
Three main problems prevent the understanding of the local structure of water by
RDF analysis. First, the result is significantly determined by the initial assumptions,
which are not yet well defined. As a result, we get significantly different results for
RDF, depending on the molecular potential that we use. Second, the distribution function reflects the time- and space-averaged configurations of short-lived local molecular environments. This fact makes the analysis of the potential-energy-minimum
F-structure controversial. For example, the averaged position for rotating pendulum
(see Fig. 1.12) corresponds to the upper point, because it spends more time there
than at any other point, while the potential-energy-minimum position is obviously
at the bottom. This analogy shows the difference between the diffusion-averaged
and potential-energy-minimum structures. Third, although the concept of interstitial
molecules qualitatively explains the denser configuration of water in comparison with
ice, it was not confirmed by high-pressure experiments [46]. The expected increase
of the concentration of interstitial molecules at higher pressure in this model has not
been observed experimentally.
Therefore, we cannot learn more about water structure using the RDF only. The
RDF confirms the roughly tetrahedral arrangement of molecules, but does not provide
the angular distribution among neighboring molecules, nor does it reflect the distribution around the local atom within short periods of time. Thus, the RDF analysis
highlights the D-structure but says nothing about the I- or V- structures [3].