A question about improper energy in Dreiding force field

The msilmp tool should generate correct improper angles for the
potentials it know about. There is no guarantee it will do so
for the Dreiding umbrella improper. Maybe the improper angle
is defined differently for that improper style?

Steve

When we prepared our unit cell for lammps using msi2lmp tool, we firstly converted to meet the format of cvff forcefield. Then we only replaced potential parameters from cvff to Dreiding.
At this process, mapping of potential parameters are exact. Thus, definition of the improper angle seems to be not affected by our approach.

From : “Steve Plimpton”<[email protected]…24…>
Date : 2011/09/20 화요일 오후 10:55:05
Subject : Re: [lammps-users] A question about improper energy in Dreiding force field

The msilmp tool should generate correct improper angles for the
potentials it know about. There is no guarantee it will do so
for the Dreiding umbrella improper. Maybe the improper angle
is defined differently for that improper style?

Steve

I think this is not problem with msi2lmp tool.
We have been using msi2lmp in various cases, but there was no problem in previous simulations using pcff or cvff forcefields.

From : “Steve Plimpton”<[email protected]…24…>
To : “Seunghwa Yang”<[email protected]…1440…>
Date : 2011/09/19 월요일 오후 11:13:39
Subject : Re: [lammps-users] A question about improper energy in Dreiding force field

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

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

read_data Rubber_dreiding.lammps05 #—> minimized structure

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
[email protected]…1440…

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Yu, Suyoung
Ph.D Candidate

Smart Structures and Design Lab.
School of Mechanics and Aerospace Engineering
Seoul National University

Tel : +82-2-880-1645

FAX : +82-2-877-1693
E-mail : [email protected]…1440…


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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
[email protected]…1440…

There are several things that could be mismatched with how
msi2lmp writes out improper interactions for use with
improper_style umbrella (since it was designed to work
with improper_style cvff). The 4 atoms could be in
the wrong order, the angle could be something like
180-theta instead of theta, etc.

Those would affect the answers you get presumably.
I suggest you stare at the output of msi2lmp (the data
file for LAMMPS input) and see if each improper
is specified the way the doc page for umbrella says
it should be. And run a test case with MSI vs LAMMPS
for a simple system with one improper and compare
energies, forces.

Steve