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I am developing an amber force field for a branched polymer, but so far it did not work.
I am looking for any source helping me to understand <HarmonicBondForce>,
<HarmonicAngleForce> and <PeriodicTorsionForce>.

Best,
Elsaid

Is there something specific you want to understand? If you're asking about the syntax of the XML file, that's described in http://docs.openmm.org/latest/userguide ... g_ffs.html. If you're asking about the functional forms of the different forces, see http://docs.openmm.org/latest/userguide ... orces.html.

I successfully add the force block for a linear one, and it works. But I can not do a modified xml file to get the pdb file of the branched polymer get accepted by the program.
I think I have to understand deeply how bonds types are written there.

For that chain:(just an example )
(CHO2)(CH3)(CH3)
Every methy group is attached to an O from the first residue
I need to have an amber force field to the above (force blocks especially)
Thanks.

What is the difference between the amber force field for linear and branched structures?

The pdb file has 2 types of residues "COR and EGH" in 1 chain "A"
I define the residues like:
<Residue>
<Residue name="COR" >
C................
</Residue>
<Residue>
<Residue name="EGH" >
C................
</Residue>

Then I am getting the error:
anaconda3/envs/opki/lib/python3.10/site-packages/openmm/app/forcefield.py", line 295, in loadFile
resName = resname_prefix+residue.attrib['name']

KeyError: 'name'
there is something wrong?