A question about improper energy in Dreiding force field

Hi there,

I`m using dreiding forcefield to characterize some properties of rubber-like
materials.

I constructed unit cell via Material studio package, then transfered it to
lammps using additional tools in lammps, “mis2lmp.exe”.
And all the Dreiding force field paramters for my simulation cell were correctly found from the original paper by Mayo and GoddardIII and assigned for my simulation.

When I compared the energy evolution in NVT and NPT ensemble observed from lammps with the result from Material studio, the improper energy and torsion energy are not the same. While, other energy terms such as pair, nonbond coincides well each other.

Especially, the improper energy of the cell from lammps is about 15970 while that from material studio is about 30. For dreiding forcefield, the improper style unmbrella was used.
Potential term of the mproper style umbrella is given as
E = K(1-cosine(w)), when initial improper angle is zero.
Of course my material have initial improper angle of zero.

And in my systems, there are 200 improper angles and the coefficient K is 40 for my system.
Thus, the maximum improper energy possibly obtained is 16000, and the minimum value is zero when each of these two cases have 180 degree and 0degree of improper angle respectively.

Again going back to my result from lammps and material studio, you can find out that the improper angle has been treated as 180degree in lammps while it has been treated as 0 degree in material studio. That is, the improper angle seems to be reated as 180-w in lammps.
But what is sure is that according to the definition of improper angle, the angle of my system has been found to be almost zero during the simulation.
Below is part of my input script.

Regards

units real
atom_style full
boundary p p p

pair_style lj/cut/coul/long 9.5 9.5
dielectric 1.0
pair_modify tail yes #mix arithmetic

bond_style harmonic
angle_style harmonic
dihedral_style charmm
improper_style umbrella
kspace_style ewald 0.0001

Seunghwa Yang, Ph.D
Post Doctoral Research Associate

Smart Structures & Design Lab
WCU multiscale mechanical design division
School of Mechanical and Aerospace Engineering
Seoul National University, Korea
Tel : +82-2-880-1645
Fax : +82-2-880-1693
http://ssnd.snu.ac.kr
fafa77@…1440…

I’m not sure if you are saying this is a problem with the improper_style umbrella
in LAMMPS that is part of the Dreiding ff, or with the msi2lmp tool assigning
the wrong angle, or if it just a difference in definition of the improper
angle between the MSI codes and LAMMPS.

I’m CCing Tod and Stephanie to see if they want to comment on the
Dreiding and msi2lmp issues.

Steve