Density, Enthalpy of Vaporization and Local Structure of Neat N-Alkane Liquids

Round 1

Reviewer 1 Report

This paper presents simulation results for liquid alkanes using the GAFF2 model. In general, the work is carefully done and the paper is clear. They mainly focus on the predicted density (as the title implies) near the experimental boiling point.  They also show results for pair correlation functions. Since a goal of this paper is to see how well the GAFF2 model does, it would be useful to look at other properties, like the enthalpy of vaporization, which is easy enough to calculate, is directly comparable to experiment, and provides a measure of the energetics of the system. (For instance, the authors claim that since the predicted densities are too high, the model underestimates repulsions, but it could be that the length scales, as established by the Lennard-Jones sigma parameter are overestimated. That is a slightly different thing from an interactions point of view.) Also, it might be helpful to look at properties at 25 degrees C, since that is often where most interest is. 

The authors extract "a free energy of binding" from the maximum of the radial distribution function. And the from the temperature dependence of that, an entropy is found. This analysis is interesting, but I wasn't sure where it came from. Has that been used before? Also, I wonder if the entropy effects seen in Figure 5(b) is not due solely to length, as the authors discuss (on page 10) but also due to temperature, since each alkane of a different length is at a different temperature (because it has a different boiling point).

Lastly, the authors might want to discuss or present results for different models. There are lots of models for alkanes out there. How does GAFF2 do relative to those?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

This is a clearly written manuscript that motivates well the study in the introduction.  A major drawback of the manuscript is that it does
not present any explanations of the simulation data. The presentation of the data and their discussion is limited to a description of the computational output. The radial distribution functions are incomplete in several ways. (1) A table should be presented with the integral under the
curves that show the number of surrounding atoms around the central atom. (2) Methane shows a long range order in RDF. Why?

The binding free energies should be explained. I do not see any binding between the hydrocarbon chains. Time correlation functions should be presented that show the relaxation times especially for the longer hydrocarbon chains. Explanations not just descriptions of the data and comparisons with other works in the literature should be added in the study. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have addressed many of my questions.