Benchmarking Plane Waves Quantum Mechanical Calculations of Iron(II) Tris(2,2′-bipyridine) Complex by X-ray Absorption Spectroscopy

Round 1

Reviewer 1 Report

This is a manuscript combining molecular level DFT structural optimization with structural solution based on comparing XANES models with XANES experiments.  It presents a very useful application and should be published.  It is very sound scientifically.  Only minor changes are needed.

1. Please provide some details on the specific model used in the simulations. How were the muffin-tin (MT) radii chosen?  What is the effect of the interstitial region since the MT approximation is used?
2. Please comment on the uniqueness of the MXAN solution of structural models?
3. Please be specific about the term “non-structural parameters”. How are they actually varied (and chosen).
4. The main peak in the XANES has three components in the simulation but two in the measured spectrum. Please comment.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript entitled "Benchmarking Plane Waves Quantum Mechanical calculations 3 of Iron(II) Tris(2,2′-bipyridine) Complex by X-ray absorption spectroscopy" by Nico Sanna and Maurizio Benfatto. It is submitted to the Special Issue "High Precision X-ray Measurements 2021". A novel computational procedure based on plane waves based on the QE package is proposed to describe the low (LS) and high spin (HS) conformational states of the complex [Fe(bpy)3]2+ in order to mimic the experimental spectra of X-ray absorption near-edge structure spectroscopy (XANES).

The manuscript is clearly organized and well written. A comprehensive review of the current techniques to compute the cross section with gaussian and TD-DFT-related 18 references is given with a broad covering of the field. The experimental data are compared with several optimized PBE/QM and PWscf geometries in detail. All computational packages, Quantum Espresso with the PBE exchange-correlation approach, ultrasoft PP, large cut-offs, MXAN software etc., provide reliable and encouraging results with the reduced error in some cases. The computed delta_EQ/S (kcal/mol) is very close to the experimental range of values.  

Section Materials and Methods is usually placed before Section Discussion.  

I can recommend this manuscript to journal Condensed Matter and selected Special Issue without further changes.  

Author Response

We are grateful to this referee for the appreciation she/he has shown for our manuscript and for pointing out to us that the Materials and Methods section was mistakenly put after the Discussion section. This error has been corrected in the new version of the manuscript.

Reviewer 3 Report

The authors report an interesting computational study, by comparing the energetics of metal complex, employing QM/DFT and PW methodology.

To my opinion the following issues may need further clarification prior to any publication:

1. It is not clear whether QM and/or PW reported data, were computed with gas or in the presence of the solvent (similar to what is reported in line 256).
2.  The symmetry of the complex, in each spin state, is given in a obscure way. What's the symmetry of the optimum geometry in HS/LS state? What's the spin (Stot) of HS/LS ?  Moreover what;s the value of the spin contamination ?
3. More details should be provided for the PW approach and the implementation of the spin state.
4. Which the more stable- HS,LS- state and geometry structure in each approach ? Moreover, comments should be provided for the electronic configuration (ordering of states) from QM and PW.
5. Please define the ΔΕQ/S equation (is it E(HS)-E(LS) ?)
6. Table 2, PBE/6-311G+G should be PBE/6-311+G ?
7.  Why the authors used the B3LYP+D (dispersion) correction ? Why not any LR functional ?
8.  More information should be given for the reproducibility of Figs 4,5. How the computational data were computed ?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The article can be published in its current version