# [gmx-users] RE: heat capacity etc. (Thomas Koller)

David van der Spoel spoel at xray.bmc.uu.se
Sat Mar 12 09:43:21 CET 2011

```On 2011-03-12 07.52, Thomas Koller wrote:
> Thanks,
>
> the equation for the heat capacity is:
>
> c(P) = delta(H(average))/delta(T(average))
>
> So do it have to take the fluctuations (which are listed with g_energy) of U(tot) and pV as well as of T to calculate c(P)? The problem is the delta.
>
> The same is with the thermal compressibility:
>
> a(P) = 1/V(average) * delta(V(average))/delta(T(average))
>
> Another question: Why should I take U(tot) and not only the potential for c(P)? The same is for the heat of vaporization, here also with U(tot)?
>
> Regards,
> Thomas
>
You need to compute the enthalpy = Etot + pV. The fluctuations in Etot
and pV are correlated and compensate each other to some extent, and
hence you need to sum the two first and compute the fluctuations afterward.

--
David van der Spoel, Ph.D., Professor of Biology
Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205.
spoel at xray.bmc.uu.se    http://folding.bmc.uu.se

```

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