Peer review of: Dissociation of salts in water under pressure (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
In this work Galli and co-workers perform a series of ab initio molecular dynamics simulations of 
Na Cl under pressure comparing the results of classical potentials to those from first principles. 
They find the existent of contact minima and solvent separated minima in the potentials of mean 
force and that this behavior is present in both ambient pressure and high pressure simulations. 
There are apparently some discrepancies between the stability of the contact vs solvent separated 
minima in the force fields compared to ab initio. 
 
Overall, this is an interesting contribution. However, I am not so convinced that there is any big 
surprise in these set of calculations that would warrant publication in Nature Communications. The 
part that I think is most interesting is the role of water dissociation on the Na+--CL- dissociation. 
This is however buried right at the end of the paper and would require significant work to bring 
that part of the paper to the forefront. In particular: 
 
i) Figure 7 left panel differs significantly from Figure 2 bottom left. The question is why? It seems 
like water dissociation is adding barriers and creating minima in all the regions of free energy 
space that weren't there before. 
 
ii) Related to the water dissociation and its role, its interesting to speculate a bit about the role of 
pH on NaCl nucleation and dissociation. Is this known/understood? Any experiments on this front? 
 
iii) Again in reference to the role of water dissociation, an examination of how the 
enthalpic/entropic contributions come from the dissociation and how they tune the total free 
energetics would be extremely interesting and would take this paper from being a more specialised 
one to something with broader impact. 
 
 
Reviewer #2 (Remarks to the Author): 
 
The first principles MD (FPMD) study of salts under high P and T has been expertly performed and 
a welcome addition to the literature. However, the accompanying classical MD studies attempt to 
serve multiple purposes, raise more questions than providing answers, and overall distract from 
the main message given by the FPMD results. The first issue with the classical force fields is that 
they are optimized at ambient conditions and should not be used at high P and T. There are 
several studies showing their failure away from ambient conditions. The main reason for this 
failure is the lack of polarization. Curiously, polarization was completely ignored in the force fields 
used in this study. Instead the effect of flexibility of water molecules was considered, which is 
rather negligible compared to polarization. Polarization has been incorporated in some force fields. 
So if the authors are keen to compare the FPMD results with classical ones, they better use such 
force fields. I also note that such comparisons carried out at different temperatures (i.e. 300 vs 
400 K) have no clout, as the results differ substantially between the two temperatures. If there is 
a problem with the accuracy of water molecules at 300 K (that is the excuse for using 400 K 
instead of 300 K in the FPMD simulations), it could also happen at 1000 K and 11 GPa. 
 
I recommend using the classical MD results only to address sampling issues, separating free 
energy contributions, etc, but not for comparison with the FPMD results. To give an example, the 
difference between the results using the SPCE and TIP3P water models is even larger than those 
affected by increases in T and P, which would make anyone very suspicious of the relevance of 
such diverse results from the force fields in the context of ab initio simulations. 
 
 
Reviewer #3 (Remarks to the Author): 

The authors present classical and first-principles (ab initio, with PBE functional) molecular 
dynamics simulations of NaCl-H2O solutions at extreme conditions with the objective of 
understanding the nature of how Na and Cl ion associate. To this end, they map-out the free 
energy surfaces for NaCl ion association and the relative energies of the contact ion pair and the 
solvent-separated ion pair. 
 
To understand the solvation of Na and Cl, they present angular distributions of the solvent 
molecules about the Na and Cl (Figure 1). Perhaps I’m missing the point, but I don’t understand 
what the angular (vs. the radial) distribution is supposed to tell us. More fundamentally, I don’t 
understand why they do not present Na-Cl radial distribution functions from unconstrained 
simulations using both classical and ab initio methods to illustrate the presence of the two 
complexes and whether the classical potentials agree with the first-principles results. This seems 
like an obvious omission. Classical (SPCE/SD) unconstrained simulations (e.g., Sherman and 
Collings, Geochemical Transactions, 3, 102-107, 2002) show that the solvent-separated NaCl ion 
pair disappears anyway at high temperature (325 ºC), as would be expected. I would have 
thought that perhaps the effect of pressure might enhance the formation of SSIP even at high T, 
but apparently not. 
 
Although the presentation of the paper could be improved, I cannot find any technical issues of 
concern. The main problem is simply that the objectives of the paper and the result of the study 
are not significant enough to warrant publication in Nature. It is not clear why we should care 
about the relative free energies of solvent-separated ion pairs vs contact ion pairs; we already 
knew such ion pairs exist. The study did not yield results that were especially unexpected and the 
implications of the results to our understanding of the upper mantle is unclear. As it is, therefore, I 
think this paper is of too-limited interest. Perhaps the authors could (given the free energy of the 
two types of complexes) calculate the resulting electrical conductivity and effective ionic strength 
of NaCl-H2O solutions under mantle conditions. That would at least give the results a bit more 
impact and put the results in a geochemical/geophysical context. Still, I think even that would not 
really warrant publication in Nature. If the authors actually calculated the activity of water as a 
function of NaCl concentration, P and T and then was able to argue that a free NaCl-H2O fluid 
could (or could not) coexist with mantle minerals at high P,T, they would have a Nature paper!