Subject: CERIUS liquid phase heat capacity calcs

From: "Dr. Lutz Ackermann" <[email-masked]>
Date: Mon, 24 Nov 1997 23:32:46 -0800

Dear Netters,

I would like to draw on your experience concerning liquid simulation. I 
have
spent the last three month experimenting (I am a novice in the field of
MM/MD simulations), trying to solve a problem, but am am not yet 
satisfied.

I am trying to reproduce isobaric heat capacity (Cp) data of single
component fluid phase systems (eg. liquid H2SO4), using the MSI 
CERIUS2_3.0
(OFF) software. MD runs (NPT ensemble) of typically
50ps(equillibration)+120ps(observation) invariably produce Cp values, 
that
are higher than the experimental ones (in general by 30 to 50 percent, in
some cases ut to a factor of 3!). This is true for:
 - more or less complex molecules (3 to 10 atoms)
 - different force fields
 - different temperatures
 - ensembles of different sizes (up to 500 molecules in the unit cell)

At the same time, other properties like the density are calculated to a
satisfactory degree of accuracy (only a few percent difference to
experimental values).

Maybe I am expecting to much, but the fact that CERIUS offers the
possibility of Cp calculations lead me to believe, that this is actually
possible (yielding reliable data, I mean). Are there any experts around, 
who
have managed to calculate realistic heat capacities of liquids  (using
CERIUS)? Or are there principal reasons, why this doesn't work? Or can 
you
think of  some silly mistake I may have made? Maybe you are aware of any
publications, that report the successful use of CERIUS in the calculation 
of
the Cp of liquids (although I was told by MSI support that such 
publications
do not exist, as "this is a tricky area"...)? Any other helpful 
information?

Thanks a lot,

Lutz Ackermann  (101,[email-masked]  /  [email-masked])