Help with Multiplicity in Improper Dihedrals

User discussions
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1

Hi everyone,

I’m using force field parameters from a paper, building the top file. I’m a bit stuck on the concept of multiplicity for improper dihedrals.

My questions are:

  1. What does multiplicity mean in the context of improper dihedrals?
  2. How do I choose the right multiplicity number for any dihedral?
    Any explaintion or links would be really helpful!

Thanks,
Yehonatan

2

Still don’t understand, even a little piece of information would be helpful.

3

Hi, it depends on the improper dihedral type. The harmonic improper (type 2) does not have multiplicity; in turn, type 4 impropers are essentially single Fourier terms identical to that of proper dihedrals, usually with a periodicity of 2 as it sets the minima to 0 and 180 degrees, as described in Gromacs reference manual.

Table 14 here is a great resource to understand the different interaction types, as different force fields use different standards (e.g. CHARMM uses type 2 impropers while Amber uses type 4).