Improper and Dihedral

Hi all

What is the difference between Impropers and Dihedrals?

Would you please explain the difference using an example?

Best Regards


A dihedral is 4 atoms in a linear chain, like the backbone of a polymer.
An improper is typically 1 atom (the out-of-plane) bonded to each
of 3 atoms (in the plane).


I like the pictures on this web site:

(As shown in the pictures, in both cases, the angle in question is
between the i,j,k plane and the j,k,l plane. For this reason, I'm
under the impression you could in principle use either dihedral forces
or improper forces interchangeably. I don't know if LAMMPS cares
whether atoms j and k are bonded together. [I guess I should try


Some of the dihedral and improper styles are
essentially the same as each other.
But some are different. LAMMPS does
not care that the atoms in the dihedral or
improper are actually bonded to each other.