Tuesday, March 6, 2018

Reviews of Random Versus Systematic Errors in Reaction Enthalpies Computed Using Semi-empirical and Minimal Basis Set Methods

We submitted this paper to ACS Omega January 31st and the reviews just came back

Reviewer: 1

Recommendation: Publish after minor revisions.

Comments:
The authors explored the CBH approach proposed by Sengupta and Raghavachari to compute the reaction enthalpy of a series of organic reactions using semi-empirical and low-cost HF/DFT as the low-level method. They also discussed the origin of errors for several cases that exhibited very large errors. The results will be very useful to the computational chemistry community, in terms of identifying effective means to compute reaction energetics and better ways to improve low-cost methods.

I have only a few minor comments:
1. It appears that dispersion correction was not included for DFTB3 and some NDDO methods. For reactions that involve very large molecules, dispersion may make a non-negligible contribution, as found, for example, for Diels-Alder reactions in the recent benchmark analysis by Gruden et al. (J. Comp. Chem. 38, 2171-2185).

2. There are several typos: line 55 of pg 2, "corrections WERE not included"; line 48 of pg 6, there is one additional "is".

3. It might be useful to report and comment on the computational cost for the different approaches. For example, PBEh-3c is still rather expensive compared to the semi-empirical methods.

4. Is there a "simple" explanation for the difference between xTB and DFTB3? For example, does the improved description of frequencies by xTB make a major difference?


Reviewer: 2

Recommendation: Publish after minor revisions.

Comments:
The authors have carried out an analysis of the performance of highly efficient computational methods for the computation of enthalpies of organic reactions using the connectivity-based hierarchy. The methods considered include DFT, HF, and a range of semi-empirical methods. The analysis is clear and some of the reported findings are indeed significant. While the good performance of DFT and HF is consistent with previous results, the lack of significant improvement with semi-empirical methods is particularly noteworthy. The paper is acceptable for publication after the authors address the following comment.

A more detailed analysis is reported for reaction 19 that is an outlier for some methods such as HF. In this system, the larger errors are attributed partly to the presence of the strained oxirane ring. Similarly, reaction 23 poses problems for some semi-empirical methods due to the presence of larger errors involving allene. In light of these observations, it may be useful to add a cautionary note to the range of problems that can be studied with such methods. I suggest a small paragraph to address the potential limitations of the inexpensive methods for such systems containing unusual bonding situations.


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