[gmx-users] Specific heat calculations using gmx energy

[David van der Spoel]

Den 2019-07-15 kl. 09:24, skrev Pragati Sharma:
> Dear all,
> 
> I am simulating a melt of polybutadiene consisting of  60 chains using OPLS
> forcefield. After 50 ns of NPT production run, specific heat is calculated
> using the command:
> 
> *gmx energy -f ener.edr -fluct_props -nmol 60 -driftcorr *
> and the value obtained is:
> Enthalpy                                 =    2010.04 kJ/mol
> Heat capacity at constant pressure *Cp    =    22807.5 J/mol K*
> 
> However when I calculate Cp from last 5 ns of trajectory the value is:
> Enthalpy                                 =    2004.05 kJ/mol
> Heat capacity at constant pressure *Cp    =    7934.66 J/mol K*
> 
> Do the Cp calculation is highly dependent on the equilibration of system or
> I am missing something because the experimental valu is ~ 100 *J/mol K, *so
> the values from simulation are also high compared to experiments.
> 
> *Thanks*
> 
Did you check whether the enthalpy makes a sudden drop in the beginning 
of the simulation? If so this will influence the fluctuations a lot.

-- 
David van der Spoel, Ph.D., Professor of Biology
Head of Department, Cell & Molecular Biology, Uppsala University.
Box 596, SE-75124 Uppsala, Sweden. Phone: +46184714205.
http://www.icm.uu.se