# Specific heat in gmx energy

**URL:** <https://gromacs.bioexcel.eu/t/specific-heat-in-gmx-energy/3118>\
**Category:** User discussions\
**Created:** [November 10, 2021, 1:37am UTC](https://gromacs.bioexcel.eu/t/specific-heat-in-gmx-energy/3118 "2021-11-10T01:37:07Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Lida](https://avatars.discourse-cdn.com/v4/letter/l/3ab097/32.png) [@Lida](https://gromacs.bioexcel.eu/u/Lida)\
**Post date:** [November 10, 2021, 1:37am UTC](https://gromacs.bioexcel.eu/t/specific-heat-in-gmx-energy/3118/1 "2021-11-10T01:37:07Z")

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GROMACS version: 2021  
GROMACS modification: Yes/No

Hello everyone,

I am trying to calculate the specific heat of CaCl2 solution by using gmx energy. I noticed that the unit of specific heat is J/(mol K). I am confused that what is the mol here? Does it refer to the mole of CaCl2 in the solution or the mole of the whole solution?

Thanks so much for your answer!

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**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\
**Post date:** [November 12, 2021, 2:21am UTC](https://gromacs.bioexcel.eu/t/specific-heat-in-gmx-energy/3118/2 "2021-11-12T02:21:06Z")

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All references to “per mole” in GROMACS units refer to “per mole of equivalent systems.”

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<div class="post-metadata">

**Author:** ![Lida](https://avatars.discourse-cdn.com/v4/letter/l/3ab097/32.png) [@Lida](https://gromacs.bioexcel.eu/u/Lida)\
**Post date:** [November 18, 2021, 4:11pm UTC](https://gromacs.bioexcel.eu/t/specific-heat-in-gmx-energy/3118/3 "2021-11-18T16:11:56Z")

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Thank you so much for the answer. In my simulation, I added Ca2+ and Cl- into a box with water. For example, if we have 1000 water molecules and 200 Ca2+ and 400 Cl-. Should I set the -nmol to 1600 (sum of water molecules and 2 ions) or 1200 (sum of water molecules and CaCl2 molecules)?

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**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\
**Post date:** [November 18, 2021, 5:52pm UTC](https://gromacs.bioexcel.eu/t/specific-heat-in-gmx-energy/3118/4 "2021-11-18T17:52:09Z")

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The `-nmol` flag is only relevant for pure systems in calculation of e.g. ΔHvap.

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<div class="post-metadata">

**Author:** ![Lida](https://avatars.discourse-cdn.com/v4/letter/l/3ab097/32.png) [@Lida](https://gromacs.bioexcel.eu/u/Lida)\
**Post date:** [November 22, 2021, 4:36pm UTC](https://gromacs.bioexcel.eu/t/specific-heat-in-gmx-energy/3118/5 "2021-11-22T16:36:59Z")

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Thank you! If so, may I ask how can we calculate the specific heat for a mixture system?

I also used gmx dos and I noticed that in the dos.log file, the number of molecules is the sum of water molecules and 2 ions.