NVT/NPT pressures w fixed particles

From: Arturas Ziemys (arturas.ziemys_at_uth.tmc.edu)
Date: Mon Oct 20 2008 - 10:03:43 CDT

Hi, all.

I have a system with a significant amount of fixed or constrained
particles. It is a fixed membrane with fixed XY cell dimensions and Y is
let to change. When I do NPT equilibration with Langevin Piston set to 1
bar, I see pressure output way off 1 bar. That is expected due to big
number of restrained particles and it is mentioned in manual. After I
have volume equilibrated at NPT, I switch to NVT ensemble. Now, pressure
fluctuates closer to target pressure, but still not as close Is should
be expected in ideal case.

Q1: Do restrained or fixed particles contribute/distort pressure the
same amount at NVT as it does in NPT ? Is anywhere some reference to
understand more about that ?
Q2: How to understand pressure values in output, if target pressure is
one, and equilibration gives another.
Q3: How to evaluate pressure in the case with a lot of restrained/fixed
particles ?

Best
Arturas

-- 
Arturas Ziemys, PhD
  School of Health Information Sciences
  University of Texas Health Science Center at Houston
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