Zhang Zhigang wrote:
Hi, all,
Actually I think others have also noticed this "bug": g_energy
often generate incorrect heat capacity. When I checked the source code
of gmx_energy.c, I found the calculation of heat capacity is only
related with the fluctuation of temperature, which is actually the
algorithm for NVE ensemble only (refer to Allen & Tildesley, Computer
simulation of liquids, pp. 53, eqn. 2.82). For other ensembles, e.g. NVT
ensemble, the heat capacity should be related with the fluctuation of
energies.
Another problem: even with the energy fluctuation algorithm for the
heat capacity, I found it is not easy to get an convergent value. The
most important factor is that the fluctuations are sensitively related
with the temperature (pressure) coupling time constants for NVT (NPT)
ensembles.
With a proper ensemble, e.g. NoseHoover, the time structure of the
fluctuations is influenced by the T-coupling, but not the amplitude,
which is what matters in the equations for heat capacity, isn't it?
I wonder if anyone can help me resolving this problem? Thanks a lot!
Regards,
zgzhang
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