AMBER Archive (2007)

Subject: AMBER: Heat capacity calculations not coming out right

From: David Cerutti (dcerutti_at_mccammon.ucsd.edu)
Date: Fri Oct 19 2007 - 21:37:38 CDT

Hello,

    I can send the relevant outputs (P, V, E, and T for each frame of the
trajectory) to anyone who might want to look at this more closely, but I'm
getting some fairly weird results for Cp values on SPC/E water. The
reason I'm doing the calculation is that I'd like to get Cp values (or,
more precisely, specific heats) for mixtures of glycerol and water to
compare to literature or possibly some of my own DSC measurements.

    When I use the fluctuation of enthalpy for a system of 1024 water
molecules at 298K, I obtain Cp = 20.1 cal/mol-K (that number stands on
about 6ns worth of trajectory). This seems to be somewhat higher than the
value previously reported by Yujie Wu in his original SPC-Fw paper (18.6
cal/mol-K).

    What concerns me more, though, is my inability to get similar results
when I compute Cp as the derivative of enthalpies for this 1024 water
system at 298K and 303K. I get convergent results there, too, but the
values I'm getting are only about 15 cal/mol-K.

    It is this latter method that I would prefer to use to compute the
specific heat capacity of the gylcerol:water mixtures, because it is so
straightforward. Also, Yujie's value of 18.6 was obtained using the
enthalpy derivative method, so our results are definitely divergent.

    The next thing I'm going to do is try to compute Cp using Yujie's
exact setup. But, meanwhile, can anyone think of what might go wrong in
this calculation?

Thanks!

Dave
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