Hello Andrew:

Thanks for replying. I am using class2 style for dihedral. The formulation

for dihedral, as you may know, is

E = SUM(n=1,3) { V(n) * [ 1 - cos(n*Phi - Phi0(n)) ] }

Here is the parameters for the specific type of dihedral I am looking at:

[K(1), phi0(1), K(2), phi0(2), K(3), phi0(3)] = [-0.2802 0.0000

-0.0678 0.0000 -0.0122 0.0000]

You are correct. If all of the phi0 parameters are zero, then the

angular distribution should be symmetric. First verify that "msi2lmp"

is generating the correct LAMMPS parameters. Can you post the

dihedral_coeff commands that msi2lmp created? (...and/or the "Dihedral

Coeffs" section from your DATA file?)

Next: it might be easier to find your problem if you have a small

system. For your convenience I am posting a small 4-atom test system

I used to test "dihedral_style class2"

Edit the "in.test" file and replace the following line:

dihedral_coeff 1 10.0 -60 0.0 -60 0.0 -60

with:

dihedral_coeff 1 -0.2802 0.0 -0.0678 0.0 -0.0122 0.0

(These are the parameters you said you are using.)

After these modifications, does the molecule behave the way you

expect? I'm reasonably confident that this small molecule should have

a symmetric distribution of dihedral angles. Please try it and

report back.

I see a bit in-consistency between the formulation used by LAMMPS and that

found in the pcff force field file. In pcff.frc, it is

E = SUM(n=1,3) { V(n) * [ 1 + cos(n*Phi - Phi0(n)) ] }

This means if pcff.frc gives Phi0(n) as 0 degree, then the parameters used

in my datafile should be 180 degree. However, in the datafile I obtained by

converting Accelrys .mdf and .car files using msi2lmp tool, Phi0(n) are all

0 degrees. Is this in-correct?

Alas, I am not familiar with the FRC file format.

In LAMMPS 0 degrees corresponds to the "cis" conformation. I do not

know whether 0 degrees in an FRC file corresponds to the "cis" or

"trans" conformation.

(terminology:

http://en.wikipedia.org/wiki/Cis-trans_isomerism

http://lammps.sandia.gov/doc/dihedral_style.html.)

Andrew

(P.S. If you know of a web site containing a description of the FRC

file format, please post it.)

test_system_class2.tar.gz (1.47 KB)