Missing parameters in cyclic peptide with Gromos FF

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GROMACS version: 2023.01
GROMACS modification: No

Dear Gromacs users and developers,

I am trying to set up a system using a cyclic peptide using the Gromos forcefields. Using the gmx_makecyclictop.pl script, everything works fine. However, I read that pdb2gmx should be able to treat cyclic peptides out-of-the-box, so I tried this approach as well. However, there seem to be missing angle and dihedral parameters in the generated topology:

[ angles ]
;  ai    aj    ak funct            c0            c1            c2            c3
    2     1     3     2    ga_18
    2     1    95     2 
    3     1    95     2 
   95     1     2     2    ga_32
   95     1     3     2    ga_31
    1     3     4     2    ga_13
    1     3     8     2    ga_13
    4     3     8     2    ga_13
    3     4     5     2    ga_15
    4     5     6     2    ga_15

[ dihedrals ]
;  ai    aj    ak    al funct            c0            c1            c2            c3            c4            c5
   95     1     3     4     1 
    3     1    95    82     1 
    1     3     4     5     1    gd_34
    1     3     8    10     1    gd_42
    1     3     8    10     1    gd_45
    3     4     5     6     1    gd_34
    3     8    10    12     1    gd_14
    8    10    12    14     1    gd_43
    8    10    12    14     1    gd_44
   10    12    14    16     1    gd_42

I assume that the bonds spanning the cycle should also have gd_… and ga_… parameters (those are defined in gromos54a7.ff/ffbonded.itp).

What is the recommended way to parameterize cyclic peptides for Gromacs? Should I still use the gmx_makecyclictop.pl script? If no, can I use the topology used by pdb2gmx, or do I need to assign the missing parameters somehow? Could this be a bug in pdb2gmx?

If that helps, I can post some example files to that others can try this out. In that case, which files should I post? (it is my first post in this forum, so I don’t know how this is usually handled).

Thanks in advance for your help.

Best,
Franz