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[lammps-users] Confused by pressure calculation
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[lammps-users] Confused by pressure calculation


From: Matthew Wander <mcfwander@...24...>
Date: Tue, 12 Sep 2017 08:42:46 -0400

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