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Re: [lammps-users] Confused by pressure calculation

# Re: [lammps-users] Confused by pressure calculation

 From: Steve Plimpton Date: Tue, 12 Sep 2017 08:32:23 -0600

The compute pressure command gives you many
options for what to include/exclude in the pressure calculation.
Using the options you should be able to figure out what
is contributing to the pressure values you are seeing,
or what other commands in your input script may be
contributing to the pressure (i.e. have a virial term).

Steve

On Tue, Sep 12, 2017 at 6:42 AM, Matthew Wander wrote:
I am doing a simple scan of a water molecule and I am a little confused by the results I am getting. The forces and positions are as follows:

Tardis:water_bend_long matthew\$ more water090_vmm.xyz

ITEM: TIMESTEP

0

ITEM: NUMBER OF ATOMS

3

ITEM: BOX BOUNDS pp pp pp

0 100

0 100

0 100

ITEM: ATOMS id type x y z ix iy iz fx fy fz

1 1 0 0 0 0 0 0 0 0.114476 0.114476

2 2 0 0 0.9578 0 0 0 0 6.11406 -0.114476

3 2 0 0.9578 0 0 0 0 0 -0.114476 6.11406

Tardis:water_bend_long matthew\$ more water095_vmm.xyz

ITEM: TIMESTEP

0

ITEM: NUMBER OF ATOMS

3

ITEM: BOX BOUNDS pp pp pp

0 100

0 100

0 100

ITEM: ATOMS id type x y z ix iy iz fx fy fz

1 1 0 0 0 0 0 0 0 0.114039 0.104498

2 2 0 0 0.9578 0 0 0 0 4.08336 -0.114476

3 2 0 0.954155 99.9165 0 0 -1 0 0.241849 4.07779

However the pressure that is calculated for the first system is:-0.0050121063
and the second is:  32.230831

Now the second makes sense. I am forcing two hydrogens together so the pressure should rise as the angle gets smaller.  The only thing I can think of is that it has something to do with the position of the atoms relative to the box corner but if this is the case then it is a real problem, that I have no idea how to fix. I've checked the forces and they are reasonable.

thanks all very much in advance
matthew

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