Xie and Long have used a diverse combination of computational methods to study the reaction of carbonyl oxide (CH2OO) with a series of aldehydes, and implemented key results in a chemistry-transport model to assess the atmospheric implications. Overall, the manuscript is well written, and the combination of methods used is appropriate to the task. I’m thus happy to recommend publication, subject to some very minor revisions. 

Questions to the authors and/or suggestions for further improvement:

1)The authors perform their most accurate calculations for reaction 1, H2COO + HCHO, and also use these results to “anchor” predictions on other reactions (as per the last term in their equation 3). However, as noted in the discussion on page 20, the computational results at 296 K are still 7.3 times higher than the most recent experimental rate. Can the authors comment on possible reasons for this? According to figure 4, the experimental and computational temperature dependence is also quite different - can this be used to narrow down the source of the discrepancy? Also, given the so-far unresolved discrepancy, the repeated use of phrases like “quantitative accuracy” , “underscoring the reliability of our calculated results” (and so on) is perhaps a bit overstated, maybe rephrase a few of these instances.

2)Please add a reference to the MW2-F12.L scheme when it is mentioned on page 7.

3)In the discussion on section 2.1., please briefly describe the main differences between W3X-L and CMMQ.L4, and if possible comment on the origin of the 0.24 kcal/mol deviation. The latter goes up to CCSDTQ while the former goes to CCSDT(Q), but given the very small CCSDTQ - CCSDT(Q) contribution this is probably NOT the major source of the deviation (as already noted by the authors on page 14 - basically I’m asking them to elaborate a bit on the “differences between the MW2-F12.L and W2X components” mentioned there). 

4)The discussion in section 2.1 mainly concerns the convergence of results with respect to the level of correlation (highest number of excitations) in the coupled cluster method. This is understandable, as this aspect is the most novel part of the work. However I note that all the “post-CCSD(T)” corrections are computed with very modest basis sets. (Again, understandable given the demonstrated rapid basis-set convergence of these corrections). Nevertheless, a brief recap of the employed basis set extrapolation (presumably performed at lower levels of theory in the MW2-F12.L scheme) could be helpful to readers. How large basis sets are used to extrapolate e.g. the HF or CCSD or CCSD(T) energies? This question is also related to point 1 above - what remaining error sources could possibly explain a discrepancy of a factor of 7.3…? 

5)”Precreation” on line 240 (page 15) should presumably be “pre-reaction”. 

6)”Barrierless barrier process” on line 340 (page 22) should presumably read just “Barrierless process”.

7)Do the “base-version model simulations” mentioned on line 420 (page 23) refer to the GEOS-CHEM simulations performed in this study, or to something else? Please clarify. 

8)Please explain why the four specific regions/areas in Table 6 were selected. Are these representative for various chemical regimes in the atmosphere, or what is the reasoning?

9)Moderate (6-12%) reductions in “gas-phase sulfate” (presumably meaning gas-phase sulfuric acid, as sulfate ions are not even stable in the in the gas phase - I do understand the phrasing may originate from the GEOS-CHEM model) were observed in Arctic/sub-Arctic regions during night due to CH2OO depletion. This is interesting - but to assess the implications better, it would be good to know what the absolute H2SO4 concentrations or production rates are in these conditions. A 10% reduction in a number that is already too small to matter is not very impactful, while a 10% reduction of a substantial number is much more important. Please elaborate on this.

Scientific significance

The order-of-magnitude difference between experimental and theoretical rate constants for the CH₂OO + HCHO reaction is a big question whose resolution is highly significant.  The theoretical rate constants presented in this manuscript clearly agree better with experiment than previous theoretical predictions (Figure 4), but it would be worthwhile to suggest at least one reason for the remaining discrepancy.

The authors make solid arguments for why CH₂OO ought to be include alongside OH and HO₂ as significant aldehyde oxidants in the atmosphere.  The prediction that fluorinated aldehydes react with CH₂OO at near the collision limit is highly significant.

The new electronic structure approaches that allow for the achievement of sub-kcal/mol accuracy for systems with eight non-hydrogen atoms is an impressive methodological advance.

The authors report the useful result that post-CCSD(T) corrections (i.e. very high levels of electron correlation) are not necessary for accurate reaction barriers as fluoro or longer alkyl substituents are added to aldehydes.

Scientific quality

On p. 4, the authors mention the common use of structure-reactivity relationships such as those proposed by Jenkin et al.  It would be good to circle back to them at some point in the manuscript, as the reliability of these estimation methods is an important issue for the atmospheric chemistry community.

Related to ways that the presentation quality can be improved (see below), the authors should briefly explain why their methodology is better than the methods of Chan and Radom.  Specifically, the authors should briefly explain why W3X-L is not sufficient for describing post-CCSD(T) effects (p. 7).  Also, why is the MW2-F12.L energy corrected stepwise by T-(T) and (Q)-T terms (equation 1), while the W2X energy can be corrected by one term from the (T) level to the (Q) level (equation 3)?

On p. 15, there is an assertion that the interconversion TS between C2a and C2b has a low barrier that leads to facile interconversion.  However, the M11L TS2_ISO 0 K enthalpy is -3.98 kcal/mol, which is essentially identical to the enthalpy of TS2d and higher than the enthalpy of TS2c.  Thus, it does not seem obviously valid to assume rapid interconversion of the two five-membered ring complexes.

There is a good justification for the use of M11L as the lower level of electronic structure theory for the direct dynamics calculations.  The authors also have performed thorough benchmarking to validate more affordable electronic structure approaches applicable to larger molecules.

The authors should briefly describe, on p. 16 or in the Supplementary Information, how they determined the number of conformers for larger aldehydes.

The authors should briefly explain how the (small) falloff factor of 1.34 (p. 18) is determined for the pressure-dependent rate constants in Table S12.

The observation that, based on their atmospheric modeling, atmospheric CH₂OO concentrations are lowered far more than aldehyde concentrations (Figure 5) is helpful.  It is also commendable that the authors refrain from overselling the atmospheric significance of their predicted rate constants.

Presentation quality

The authors provide a solid, concise introduction that does a good job of motivating the research.

The authors assume too much familiarity with the theoretical details of their research.  This makes parts of the current version of the manuscript not approachable for the large majority of atmospheric chemists who read Atmospheric Chemistry and Physics.  I recommend the following improvements:

• Brief definitions and/or explanations of topics like anharmonicity, re-crossing, the dual-level strategy for calculating rate constants, and the multi-structural anharmonic factor (equations 5 and 6).
• There needs to be a literature reference for the MW2-F12.L scheme, along with a brief description of what the scheme achieves.
• Equation 2, which presents the basis set extrapolation scheme, must be unpacked; the notation is obscure.

There appears to be an error in the color scale for the rightmost map in Figure 5.  I think the color scale is reversed: most of the world appears white, which should mean that there is virtually no change in predicted sulfate concentration in spite of the predicted faster rate constant for the CH₂OO + HCHO reaction.