c_p - isobaric molar heat capacity

User discussions
1

Hi all,

When using “gmx energy” to calculate the molar enthalpy H and the temperature T, with option “-fluct_props”, we obtain the molar heat capacity c_p. I have two questions about this output c_p.

  1. Equation?
    How do GROMACS calculate the molar heat capacity c_p from the molar enthalpy H and the temperature T? Is it by this equation?

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with

N = number of molecules [dimensionless]

image.png

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and the gas constant

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  1. Inconsistency with the physical equation?
    By physical definition, the molar heat c_p is derived from the heat capacity C_p as

image.png

with

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Thus, to put it physically accurate,

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since
image.png, thusimage.png; andimage.png.

where N = number of molecules [dimensionless].

Assuming that the equation I proposed in the first question is correct, then comparing it with the equation of c_p in my second question, there is apparently an inconsistency.

GROMACS mentioned that “There is no distinction between Boltzmann’s constant k and the gas constant R…” (GROMACS Documentation, Release 2019, section 5.3.2 MD units, p. 300, http://manual.gromacs.org/documentation/2019/manual-2019.pdf).

But by physical meaning, they are different. I cannot make sense of it, and get stuck at this inconsistency. May someone please help explain?

Thanks a lot, in advance!
I’d greatly appreciate your guidance.

Anh T. Mai