[gmx-users] Heat capacity calculations from fluctuations and using g_dos

[Srinath Kompella]

Hello all,
              I've simulated Methylcyclohexane (MCH) in NVT and NPT
ensembles using GROMACS-5.1.4, from which I want to calculate the heat
capacities (Cv and Cp respectively) of MCH. If I use the fluctuation
properties, I get the following note: "Heat capacities obtained from
fluctuations do *not* include quantum corrections. If you want to get a
more accurate estimate, please use the g_dos program." After going through
a few discussions on GROMACS mailing lists, I used "gmx dos" module
thinking that I would get the quantum corrections for the heat capacities.
But the heat capacity values written at the end of the "dos.log" are larger
than that of those calculated using fluctuation properties.

*Temperature(K)                             Cv (J/mol K)
                          Cp (J/mol K)              *
                                *Fluctuation Properties          g_dos  *
        *Fluctuation Properties        g_dos  *
       298.15                            165.564                 275.747
                 193.939                  274.531

              I noticed that the numbers fluctuation properties give are
relatively much closer to the experimental values.

              Please help me understand why the numbers are very different
and which method to go for. Thank you in advance.

-- 
Srinath Kompella
Ph.D. Research Scholar
Molecular Simulations Laboratory,
Chemistry and Physics of Materials Unit (CPMU),
Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR),
Jakkur - 560 064, Bangalore,
Karnataka State, India.