[gmx-users] pressure in NPT and NVT

[Justin A. Lemkul]

Nilesh Dhumal wrote:
> Hello,
> 
> I am using NPT simulation for water - spce model (opls-aa force field).
> I have the reference pressure set at ref_p = 1.0 bar.
> I checked the average pressure using g_energy and it is 2.81972 bar.
> 
> Why there increase in pressure?
> 
> 

Probably the system is not yet equilibrated.

> 
> For NVT simulatin I get the value around average pressure 710.685 bar.
> 
> I am running the simulation at 295 k and  average temp. is 294.98 K which
> look ok.
> 
> 
> Can you tell why there is increase in pressure for NPT and  why there is
> so high value of pressure in NVT.
> 

Because the box is fixed.  It cannot respond to internal forces in your system. 
  Pressure values during NVT don't mean much.

-Justin

> I am using gromacs version 4.0.7.
> 
> Thanks
> 
> Nilesh
> 

-- 
========================================

Justin A. Lemkul
Ph.D. Candidate
ICTAS Doctoral Scholar
MILES-IGERT Trainee
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080
http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin

========================================