[lammps-users] COMPASS question

Hi, Bruce,

The COMPASS force field is commercial. I think it won't be easy to hack it although I think it's possible.

Ting

As Ting said, COMPASS is commercial, so we don't distrbute
its FF files with LAMMPS. If you have the FF files from your
own version of Acelerys, then you can use them with the lammps/tools/msi2lmp
program to create input files for a LAMMPS run.

Steve

Hi Steve, Bruce

I’ve attached a COMPASS forcefield file that we put together from the published literature. This works with msi2lmp, as you mention, Steve. The commercial version of COMPASS is, I believe, encrypted, so I doubt that customers of Accelrys can use it with LAMMPS in any simple way. Also, I have no idea whether the Accelrys license would permit such usage.

You also need to be careful with the handling of the 1-4 nonbonds, etc. We use a section like this:

units real
atom_style full

pair_style lj/class2 9.5
pair_modify tail yes
bond_style class2
angle_style class2
dihedral_style class2
improper_style class2
special_bonds lj/coul 0.0 0.0 1.0 dihedral yes

Finally, be cautious about the definition of 6-12 or 6-9 parameters in general. Some people use rmin, while others use r0, the point where the curve crosses 0. I believe that LAMMPS handles it one way (rmin) for the pairstyle lj/class2 and the other way (r0) for pairstyle lj96/cut. It is documented – the equations are given correctly – but it is easy to make a mistake in this area!

Paul

Steve Plimpton wrote:

compass.frc (64.8 KB)

In Accelrys' product COMPASS is encrypted for obvious reason.
But for many other FF, there are in plain text format. You can verify
them by checking against the literature.

msi2lmp tool is not always reliable. My feeling is it has not been updated
for a long period of time. For example, if you convert polythiophene from
car/mdf to lammps data file using cff91, you are most probably also experiencing
missing parameter problem as we found, all the bond-bond-13 force constants
are zero which has been verified that should not be true. Double check is
necessary when you use this tool, even there is no warning.

Ting (KULeuven)