Zuzana Benkova wrote:
Dear GROMACS users,
most likely I am asking a trivial question but I need to be sure about
it for calculations of heat capacity based on the fluctuations of energy
in canonical ensemble. I am doing simulation of water using TIP4P model.
I compare two systems consisting of 216 and 8*216 = 1728 molecules. The
expected fluctuations for the larger system should be reduced
8^(1/2)=2.83 times with respect to the fluctuations of the smaller
system. Actually, this applies only to the fluctuations in intensive
properties such as the temperature, pressure and density while the
fluctuations in extensive properties such as energies exceed about 2.83
times the corresponding fluctuations for smaller system. Could you
explain me the inverse ratio of fluctuations?
Have you simulated long enough that you can measure these fluctuations
reliably? Look at the trend in their behaviour with increasing time.
I have also one question regarding the heat capacity calculation. If I
want to apply the formula for canonical ensemble (<E^2>-<E>^2)/(kbT^2)
than the capacity should be size dependent. I am a bit puzzled about it
and any explanation would be very helpful for me. Thanks in advance
Heat capacity is extensive. Quoting the output that's confusing you
would help you get better feedback.
Mark
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