Dreading Atom Types

Hello Users,

I was unsure where to ask this as this is not directly a LAMMPS question so I apologize. I’m putting together a topography parser that will assign Dreading atom types and then give a LAMMPS data file. I’m a little confused about the use of the C_R (resonance carbon) atom type. I know it would be used for all the atoms in benzene or aromatic rings, but is it also the used for all the carbons in 1,3-butadiene, or would butadiene use the C_2 (SP2 carbon) for all the carbons? I’ve tested both situations and found that bond lengths do not correspond well with several other force fields (UFF, MMFF, MM3) however, I did find by setting carbon 1 and 4 to C_2 and carbon 2 and 3 to C_R that bond lengths and angles better matched actual experimental and force field data.

Thanks for your time and again I apologize for the off topic.

Tod may be able to answer (CCd).


John (Doe??):

The polyenes are a special case. For such systems, e.g., 1,3-butadiene, the four carbons are all sp2

and would be considered as C_2 in Dreiding. However, the bond lengths are clearly quite different. In order to
handle such cases, the you would have allows different force constants for different bond orders:
consider a system
Then you would specify 2 force constants K1 and K2 for the single (a-b and c-d) and double (b=c) bounds respectively,
i.e. in LAMMPS nomenclature, bonds a-b and c-d (C_2) would have a different bond type than bond b=c (C_R) as you