[gmx-users] Heat capacity (C_{v}) for a solid

[Alexander Alexander]

Dear Gromacs user,

I am trying to calculate the heat capacity at constant volume (C_{v}) in
different temperature for a solid structure with "A_{3}B" chemical formula.
To do so, I made a big supercell containing totally "43904" atom by keeping
the rate (#A = 32928 and #B=10976), below is a part of my topology file.
topol.top:
-------------------------------------
[ moleculetype ]
; Name            nrexcl
  A                       1

[ moleculetype ]
; Name            nrexcl
  B                        1

[ molecules ]
;mol_name number
   A    32928
   B     10976
-------------------------------------

After convergence of the MD simulation I invoked the below command:

"gmx energy -f case.edr -nmol 43904 -fluct_props -o case.xvg"

My first question:
what exactly I should use as "-nmol" in this specific case as explained
above? If it is (#A) or (#B) or (#A+#B)?

Second question:
I was wondering if the Cv printed out in screen after choosing "Total
Energy"  and "Temperature" in gmx energy is relabel?  At least in this
level as I know the Quantum part has not been included yet.  Or I have to
do the temperature numerical derivation of Total energy myself as
Cv=d(U)/d(T).

WARNING: Please verify that your simulations are converged and perform
a block-averaging error analysis (not implemented in g_energy yet)
Heat capacity at constant volume Cv   =    200.779 J/mol K.

Third question:
If this is Molar heat capacity, am I right?

The last:
Below are the result of Cv in different temperature for this system:

(Cv  J/mol K) ----- (Temperature K)
37037.700                      10
     200.779                      50
       40.497                      75
     124.360                      100
     321.416                      150
     620.247                      298

I know these Cv do not include any corrections for quantum yet, but still I
am puzzled about the strange behavior of  Cv versus temperature, I would be
so thankful if one could explain it for me.

(Version of Gromacs : is 5.1-rc1)

Sincerely,

Alex