`No default Improper Dih. types` for ACE-PRO-NME peptide with charmm36-jul2022.ff

GROMACS version: 2022.5
GROMACS modification: No

How to reproduce an error.

What I did

I tried to execute the following command:

gmx grompp -f ions.mdp \
           -c solv.gro  \
           -p topol.top \
           -po mdout_ion.mdp \
           -o ions.tpr

but I got the error:

ERROR 1 [file topol.top, line 274]:
  No default Improper Dih. types


Excluding 3 bonded neighbours molecule type 'Protein'

Excluding 2 bonded neighbours molecule type 'SOL'

There was 1 note

-------------------------------------------------------
Program:     gmx grompp, version 2022.5
Source file: src/gromacs/gmxpreprocess/grompp.cpp (line 2172)

Fatal error:
There was 1 error in input file(s)

For more information and tips for troubleshooting, please check the GROMACS
website at http://www.gromacs.org/Documentation/Errors
-------------------------------------------------------

For more detail, I give the file error.log in the Google drive directory.

I am using charmm36-jul2022.ff.tgz and the system consists of ACE-PRO-NME.
Also, I noticed that the script succeeded in passing other amino acids and terminated normally.
This may imply proline-related issue, I suppose. But I don’t know what to fix this problem.

Could you possibly please help me to solve this issue?

Thanks in advance.

I found a way to work around the error, which is

Remove the line 274 in topol.top.

The line corresponds to atom ids for a dihedral angle.
When I view the atoms in PyMOL, I found them weird because a hydrogen was not connected to the other three atoms defining the dihedral angle (See the figure below):

Nonetheless, I don’t know why this took place… Probably, proline capped by ace and nme may not be processed normally by gmx pdb2gmx with the charmm force field, for some reason.

Encounter with same error for my proline rich peptide (pdb:-1CKA)
Did you find any way.

Unfortunately, not…

So I just deleted the dihedral line which cause the error
Is it correct.
Then it’s running perfectly and got the correct results
And also binding at the correct position.