Peer review of: Formation of ammonia–helium compounds at high pressure (2020)

REVIEWERS' COMMENTS: 
Reviewer #1 (Remarks to the Author): 

The major claim of this paper is that there exists solid solutions of ammonia and helium at the 
pressure and temperature relevant to Ice Giants such as Neptune and Uranus and that it can support 
a super-ionic phase. The study is well constructed although somewhat derivative in that they 
basically follow the outline given in Bethkenhagen et al. (2015), i.e. a combination of structure 
search, quasi-harmonic free energy and molecular dynamics simulations to identify super-ionic 
behavior of various phases as a function of pressure and temperature. Be that as it may, the system 
that they considered is relevant to planetary science and this contribution will make an impact on 
the field. However, I would point out that some models of the planetary structure reproducing the 
gravitational moments of the Ice Giants already considered adding a helium (and hydrogen) 
component in the outer core of the planets [Figure 9 in N. Nettelmann, K. Wang, J. Fortney, S. 
Hamel, S. Yellamilli, M. Bethkenhagen, and R. Redmer, Icarus 275, 107 (2016)] since a lower density 
than what could be provided by the equation of state of water ammonia and methane mixtures was 
required to fit the gravitational moments of Uranus. While the authors point to that reference, their 
study lends support to that assumption and they should discuss this aspect in more detail. The 
computational details to reproduce the work are sufficient and clearly laid out. 
 
Reviewer #2 (Remarks to the Author): 

This manuscript describes the results of employing currently the Calypso code, the VASP code, and 
the Phonopy code to predict the structures and selected properties of possible helium-ammonia 
(He-NH3) compounds. 

The authors suggest that the predicted compounds are candidates for materials in the planets 
Neptune and Uranus. These computer codes employed are currently considered to be among the 
state-of-the-art codes for this type of study and the use of the computer codes as employed by the 
authors is considered to be standard practice for structure searches and characterization. All 
parameters employed for each code are adequately described and correctly chosen in this 
reviewers’ opinion. 

The molecular dynamics simulations that were also carried out to explore the superionic property of 
(NH3)2He compound at high pressures and high temperatures are clearly described. 

The characterization of He compounds is also correctly aided with the use of zero-point vibrational 
energy (ZPE). The Bader analysis discussion adds interesting detail to the characterization of the 
predicted compounds. 
 

My specific comments for the authors to address and make appropriate minor modifications are 
listed below. 

1) There is a sentence on page 4 that needs correction. It now is written as “: ”only H atoms in 
(NH4)+ unit diffuse around N atoms limited in a local space.” This needs to be clarified. 

2) The total time for the ab initio MD should be added along with the time step already listed. 

3) The reference list should be brought up-to-date. For example the reference “Peihao Huang, Hanyu 
Liu, Jian Lv, Quan Li, Chunhong Long, Yanchao Wang, Changfeng Chen, Russell J. Hemley, and 
Yanming Ma, PNAS March 17, 2020 117 (11) 5638-5643; H3O in Neptune and Uranus” should be 
referenced and commented on. 

4) The reference number 37 in the manuscript is incorrect and incomplete. 

5) Can the authors make any predictive comment on the relative stability of compounds such as 
those references in references 36, 37, and that of the 2020 references mentioned above? The 
answer to this question may rely on future studies as suggested by the authors but any comments 
that the authors could make in the manuscript would be welcome.

6) The manuscript contains too many minor English and grammatical deficiencies to be listed here. 
Another careful reading will be needed to eliminate these deficiencies. 

In summary, although no new methods are developed, the methods employed are state-of-the-art 
so the results can definitely be considered to be reliable. To fully understand the composition of 
Neptune and Uranus, the authors have correctly employed current theoretical methods. Studies 
such as the one described in this manuscript provide new pieces of essential data to eventually 
characterize the composition of these planets. This manuscript may therefore provide a basic 
reference for expected future characterization studies of Neptune and Uranus.