Rigid bugs

I have been doing simulations with rigid water models, and have been having some issues with the fix rigid and related commands. First of all, I cannot seem to fix the temperature separately from the rigid command, i.e. if I use the commands

fix 1 all rigid molecule

fix 2 all nvt temp 298.0 298.0 100.0

the results are very strange (like the radial distribution function for bulk water for example is erratic and completely wrong). If I use the rigid/nvt command, the results are normal

fix 1 all rigid/nvt molecule temp 298.0 298.0 100.0

However, I want to use the rigid/small command, because supposedly it is faster for many small molecules. The rigid/small/nvt command is not actually supported, so I would like to run something like

fix 1 all rigid/small molecule

fix 2 all nvt temp 298.0 298.0 100.0

But as I mentioned, this gives very strange results.

It is not suppose to use rigid body with normal integrator.

Yi

Did you use rigid/nvt/small?

Yi

I have been doing simulations with rigid water models, and have been having some issues with the fix rigid and related commands. First of all, I cannot seem to fix the temperature separately from the rigid command, i.e. if I use the commands

fix 1 all rigid molecule
fix 2 all nvt temp 298.0 298.0 100.0

the results are very strange (like the radial distribution function for bulk water for example is erratic and completely wrong). If I use the rigid/nvt command, the results are normal

fix 1 all rigid/nvt molecule temp 298.0 298.0 100.0

However, I want to use the rigid/small command, because supposedly it is faster for many small molecules. The rigid/small/nvt command is not actually supported, so I would like to run something like

Fix rigid/small/nvt is available in the current version of LAMMPS. Please check the online documentation.

fix 1 all rigid/small molecule
fix 2 all nvt temp 298.0 298.0 100.0

But as I mentioned, this gives very strange results.

You would have to use a non-time-integrating thermostat for this.

Hi all,
Thank you for the quick responses.

Sorry, I did write rigid/nvt/small, sorry about that typo. I do see it in the documentation, but it doesn’t work when I try it, and is not listed in the “style_fix.h” file. I am using lammps-1Feb14 distribution, so I think it is fairly recent.
Can you explain what you mean by non-time-integrating thermostat and normal integrator? Thanks.

This may also be related to another bug I encountered. I found that using a Berendsen thermostat gave very strange results earlier. Is that a non-time-integrating thermostat? Perhaps I need to understand this difference.
-Teddy

Hi all,
Thank you for the quick responses.
Sorry, I did write rigid/nvt/small, sorry about that typo. I do see it in the documentation, but it doesn’t work when I try it, and is not listed in the “style_fix.h” file. I am using lammps-1Feb14 distribution, so I think it is fairly recent.

No. Plenty of updates since then.

Can you explain what you mean by non-time-integrating thermostat and normal integrator? Thanks.

Please reread the manual. It explains these things in detail.

This may also be related to another bug I encountered. I found that using a Berendsen thermostat gave very strange results earlier. Is that a non-time-integrating thermostat? Perhaps I need to understand this difference.

Berendsen is has problems as is documented in the published literature and also explained several times on this mailing list.

There are many reasons to get “funny” results.