[gmx-users] Different Cv and Cp

Michael Shirts mrshirts at gmail.com
Mon May 25 16:11:31 CEST 2015

Are you running with the Berendsen thermostat or barostat?  The gromacs
g_energy functions for heat capacity use the fluctuation formula, and the
fluctuations with both of these algorithms are wrong (as should be printed
in the log file warning message). Make sure you use ensemble-preserving
thermostats if you want fluctuation properties.

Also, to be sure, you should double check by calculating both heat
capacities by finite difference formulas as well with two simulations at
T+dt/2 and T-dt/2 -- if the fluctuation and finite difference resutls don't
agree within propagated error, then something is off.

On Mon, May 25, 2015 at 5:59 AM, Faezeh Pousaneh <fpoosaneh at gmail.com>

> Hi,
> I do not know why I obtain two difference cp and cv from NVT and NPT
> simulations.
> What I do is, I take 1000 lutidne molecules, and I do firstly an energy
> minimization with steep integrator, then NPT simulation at T=300 and P=1
> atm for 10ns, (I obtain Cp= 230), then I run NVT for 10 ns with same mdp
> file except no pressure coupling, and with initial .gro file obtained from
> NPT run, (I obtain Cv=180).
> Does some one know where is my mistake? (In both runs, I obtain Cv and Cp
> from g_energy and in different time intervals and after equilibrited time)
> Best regards
> --
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