Atomic Structure, Electronic and Mechanical Properties of Pyrophyllite under Pressure: A First-Principles Study
Round 1
Reviewer 1 Report
I’m wondering what could be the reason that lead the authors to compute the pyrophyllite elastic properties using the LDA functional, while they could use the far superior PBE-D2 functional at the same computational cost. In fact, the LDA functional gives a worse description of chemical bonding, and has for some time employed to simulate wan der Waals solids because it simulates in some way the presence of dispersion forces, even though in an unphysical way. Actually, the PBE-D2 lattice constants are in a much better agreement with the experiment than the LDA ones herein reported (see e.g. Belzunces et al. Journal of Computational Chemistry 2017, 38, 133, not cited here). It is likely that the elastic constants are also influenced by this choice, as for sure is the description of the bonding properties.
Comment:
0) Please, justify the use of the LDA functional (see above).
1) The manuscript is poorly written. In particular, there is an extensive use of the past tense in place of the present tense (e.g. “The unit crystal layer was a piece of…”)
2) line 74: “vienna” à “Vienna”
3) line 168: “on increasing the pressure the distances between atom gradually shrink”: predicting the opposite would be very surprising.
4) lines 175-179: A shorter bond length does not mean a stronger bond when different couples of atoms are compared. Furthermore, I would say that the easier substitution of Al ions is related to the higher ionicity of the Al-O bond with respect to the Si-O one.
5) line 222: “more incompressible” à “less compressible”
6) line 248: “calculated by VASP software” à “calculated by LDA”
7) Table 5 is unreadable, and duplicates the information of Figure 4, which is also unreadable because of the picture size.
8) Line 312: I’m pretty sure that any reader of Minerals knows that the vertical axis of a band plot refers to the electronic energy.
9) Line 342 “the pyrophyllite still has a good stability under high pressure”: I cannot understand how the authors deduce this conclusion.
10) Lines 349-362 This paragraph is a collection of trivial speculations.
Author Response
Dear Reviewer 1
Thank you very much for the helpful and useful comments and. We have extensively revised our paper to accommodate your suggestions and comments.
Our relevant reply can be found in the appendix word document.
Wish you all the best.
Best regards,
Xinzhan Qin, Jian Zhao, Jiamin Wang, Manchao He
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript reports the structural and electronic properties of pyrophyllite at different pressures. It would be nice to comment in conclusions on how the current research can contribute to geophysics or mechanics for general readers.
Another suggestion is, in the manuscript, the pressure was determined by using a series of volumes (1.0Vo, 0.88Vo....). It seems like the fractional volume was obtained by reducing isotopically the crystal volume obtained at 0 GPa. This may work in cubic structures. However, as indicated in Table 1 with the large error for the c axis compared to the a or b axis lattice parameter, the structure is highly anisotropic; Van der Waals forces between the 2:1 layers along the c axis. Errors in the reported data, due to the applied approach, should be discussed.
Two different stabilities are discussed: mechanical and electronic. Clearer explanation is required as to the concept of stability, in particular, how the electronic band structure can discuss the stability; how they are helpful info in geophysics and mechanics.
Author Response
Dear Reviewer 2
Thank you very much for the helpful and useful comments and. We have extensively revised our paper to accommodate your suggestions and comments.
Our relevant reply can be found in the attachment.
Wish you all the best.
Best regards,
Xinzhan Qin, Jian Zhao, Jiamin Wang, Manchao He
Author Response File: Author Response.docx
Reviewer 3 Report
Review attached
Comments for author File: 
Comments.pdf
Author Response
Dear Reviewer 3
Thank you very much for the helpful and useful comments. We have extensively revised our paper to accommodate your suggestions and comments.
Our relevant reply can be found in the attachment.
Wish you all the best.
Best regards,
Xinzhan Qin, Jian Zhao, Jiamin Wang, Manchao He
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The authors have substantially improved the quality of the manuscript. However, they have addressed the main criticism, i.e. the choice of adopting the LDA functional, only in the reply letter, and not in the revised manuscript.
In the letter, they state that “Historically, for layer-structured materials such as graphite, kaolinite, it has long been found that the LDA calculations are more reasonable”. However, science evolves, and one should not rely on historical approaches, but rather adopt state-of-the-art approaches, especially when they are not more computationally demanding than the old ones.
Furthermore, the authors state that “In the following work, we will test the PBE-D2 results of the electronic and mechanical properties of the clay minerals.” Why not publishing LDA and PBE-D2 data together? Writing serial manuscripts is not a good practice.
Author Response
Dear Reviewer 1
Minerals
Thank you very much for the helpful and useful comments. We have revised our paper again to accommodate your suggestions and comments.
Our response to your comments are detailed in attachment.
Wish you all the best.
Author Response File: Author Response.pdf
Reviewer 2 Report
Authors seem to address all the questions and comments of the reviewer, except one (#point 2) as to the volume reduction method representing the applied pressure. Because the crystal structure of pyrophyllite is aniostropic, isotropic reduction of the volume may cause unwarranted errors. It would be important to discuss the volume reduction method and possible errors.
Author Response
Dear Reviewer 2
Minerals
Thank you very much for the helpful and useful comments. We have revised our paper again to accommodate your suggestions and comments.
Our response to your comments are detailed in attachment.
Wish you all the best.
Author Response File: Author Response.pdf
Reviewer 3 Report
Dear Authors,
Thank you for making these changes. The revised version of the manuscript has been greatly improved.
Author Response
Dear Reviewer 3
Minerals
Thank you very much for your affirmation.
Wish you all the best.
