[gmx-users] Wrong heat capacity (water)

[David van der Spoel]

Andreas Kring wrote:
> Hello all,
> 
> I ran a MD simulation of TIP4P water in the NVT ensemble (976 molecules, 
> density=0.98 g/cm3, T=300K (Nose-Hoover), SETTLE and PME enabled). This 
> works out fine, i.e. I get a correct radial distribution function 
> (g_OO), but the heat capacity (C_V) is wrong. I calculated it using the 
> command:
> 
> $  g_energy -f ener.edr -ndf 6 -nmol 976
> 
> and get the value below (which sould have been approximately 75 J/(mol*K)).
> 
> Is the command above incorrect?'
Not necessarily. It would be good if you check the following things:

- is the simulation in equilibrium?
- how large are the fluctuations for the equilibrium part?
- is the density correct?

You can compute the cV from the fluctuations manually as well. Does that 
match the value computed by the program?

> 
> Kind regards
> Andreas
> 
> 
> 
> Energy                      Average       RMSD     Fluct.      Drift 
> Tot-Drift
> ------------------------------------------------------------------------------- 
> 
> Potential                  -41.1524   0.478018          0   -3.69759 
> -36.9796
> Kinetic En.                 7.48322   0.282052          0   -4.33977 
> -43.4021
> Total Energy               -33.6692   0.604335          0   -8.03739 
> -80.3819
> Temperature                 300.006    11.3076    11.2959  -0.178262 
> -1.7828
> Heat Capacity Cv:     -3.94726 J/mol K (factor = 2.77307)
> 
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-- 
David van der Spoel, Ph.D.
Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205. Fax: +4618511755.
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se