Fwd: pressure in interfacial tension

From: Kowsar Khajeh (khajeh.k.2005_at_gmail.com)
Date: Sun Jan 28 2018 - 21:18:39 CST

Hello dear Namd user
I am going to calculate the intefacial tension for Heptane-water interface
by using this equation; {(((Pxx+Pyy)/2)-PZZ)*Lz}. I have applied NPzAT
ensemble by activating the constantarea command.
As we know in namd output file, 4 different kind of pressure values
(PRESSURE? GPRESSURE? GPRESSAVG? PRESSAVG?) have been written.
based on namd forum; "Pressure values are in bar. Temperature values are in
Kelvin. PRESSURE is the pressure calculated based on individual atoms,
while GPRESSURE incorporates hydrogen atoms into the heavier atoms to which
they are bonded, producing smaller fluctuations. The TEMPAVG, PRESSAVG, and
GPRESSAVG are the average of temperature and pressure values since the
previous ENERGY output; for the first step in the simulation they will be
identical to TEMP, PRESSURE, and GPRESSURE.".
Honestly, it is not clear for me which pressure should be used to calculate
the interfacial tension. I checked all pages in namd mailing list about
this topic but there was not an explicit answer. I checked all of these
pressure and tried to calculate the interfacial tension. I could not get
the exact answer. There was an strange (for me) comment in mailing list
which offered to set temperature about 0.005 kelvin to remove fluctuation
in Pzz and get the exact interfacial tension. I think may be it works but
how we can validate the result. this small value for temperature is far
away from reality!!!
http://www.ks.uiuc.edu/Research/namd/mailing_list/namd-l.2009-2010/0250.html
http://www.ks.uiuc.edu/Research/namd/mailing_list/namd-l.2008-2009/4112.html
Another question is that what is the difference between pressure profile
and former type of pressure in output file?
Thanks so much for any help anyone can offer, and best regards,
Kosar

Kosar Khajeh
=================================
PhD Student
Faculty of Mechanical Engineering
University of Tabriz, Tabriz, Iran

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