[gmx-users] tau_p

[Oliver Beckstein]

> >                         I done MD for my sistem ( CL- NA+ 330Water)
> >at P=cte.
> >; Isotropic pressure coupling is  on
> >Pcoupl              =  berendsen
> >Pcoupltype          =  isotropic
> >tau_p               =  1.0
> >compressibility     =  4.5e-5
> >ref_p               =  1.0
> >The presure is really bad, very bad (average -0.5 bar, after 100ps)
> >I change tau_p= 0.5, then I obtain less fluctuacions.What is the
> >right value(s) of tau_p?.
> >
> 100 ps to average the pressure is really too small. Try at least 1 ns
> ... should be better.

For your information, after 100ns of 1M NaCl (39 Na+, 39 Cl-, 2102 water) 
I obtain

Energy                      Average       RMSD     Fluct.      Drift  
Tot-Drift
-------------------------------------------------------------------------------
Pres-ZZ (bar)               1.05953    347.934    347.932 4.24344e-05    
4.22362

(I did not use isotropic P-coupling but fixed x and y to be able to
compare to other simulations which required semiisotropic P-coupl;  for
bulk I'd normally also do isotropic as you did. I used SPC water and the
ion params from ffG43a1; PME electrostatics, tau_p = 1.0, same ref and
compressibility as above).

Thus after sufficiently long time the average converegs on the preset
value, more or less. The fluctuations are still huge, of course.

Oliver

-- 
Oliver Beckstein * oliver at biop.ox.ac.uk
 http://sansom.biop.ox.ac.uk/oliver/