[gmx-users] Pressure coupling and tau_p values

[toma0052]

Hello,
     I am simulating a lipid bilayer system under a shear stress, in which I
am interested in looking at the surface tension.  I have done a number of
simulations in an NPT ensemble using semiisotropic Berendsen pressure
coupling.  For a system such as this, I am wondering what would be an optimal
tau_p because I seem to be getting slightly different results dependent on
the value I choose.  For example, a 3ns simulation using a tau_p of 0.5 I get
a surface tension of 30 dynes/cm and using a tau_p of 5.0 I get a surface
tension of 25 dynes/cm.  Is this difference negligible?
     I have looked into this a bit, and I have seen a number of publications
in which people have used Berendsen pressure coupling in a lipid bilayer
system with a coupling constant of 0.5 or 1.0ps.  However, in the mailing
list, there are some posts in which it is recommended that one use a tau_p of
5.0ps to 10.0ps due to semiisotropic coupling not having the averaging effect
of isotropic coupling and using a small coupling time can add excessive noise
to the system.  Therefore, is it more reliable when simulating a lipid
bilayer system to use a short tau_p ~0.5ps or a longer tau_p ~ 5.0ps? 

Thank you,
Mike Tomasini