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Hello all
I am working with a pdb that contains only the atom sulfur forming a structure to be used in a simulation. I checked CHARMM 36 and it appeared as if it could handle the sulfur atom but when I perform this command:
gmx pdb2gmx
I get the error “residue ‘S’ not found in residue topology database”
Is this because it is alone? How do I trick gromacs to make this acceptable?
I don’t see an easy way to do this necessarily because you will have to modify charges of the S and any atoms to which it is bonded, which means creating several different (new or modified) residues, then modifying specbond.dat to recognize multiple bonds that act as cross-links. If it sounds too difficult, it’s probably because it is, and I do not think it is sensible to think of this S as some isolated species.
You need to create a fully modified residue that includes the S-containing prosthetic group, so you will need to parametrize and introduce one residue following the steps listed in the link above.
I worked through adding some parameters obtained via swiss param for iron and added them to these files:
to aminoacids.rtp
[ FE ]
[ atoms ]
FE FE 0.00 0
to atomtypes.atp
FE 55.847000
to ffnonbonded.itp
FE 26 55.8470 0.0 A 0.115817 0.000000
added to residuetypes.dat
FE FE
But when I run pdb2gmx I get this error.
Problem with chain definition, or missing terminal residues.
This chain does not appear to contain a recognized chain molecule.
If this is incorrect, you can edit residuetypes.dat to modify the behavior.
8 out of 8 lines of specbond.dat converted successfully
Program: gmx pdb2gmx, version 2020.1-Ubuntu-2020.1-1
Source file: src/gromacs/gmxpreprocess/resall.cpp (line 557)
Fatal error:
Residue ‘FE’ not found in residue topology database
Please copy and paste the whole screen output; this ending fragment is not enough. There’s lots that pdb2gmx does that may seem cryptic but is really important and potentially diagnostic. Upload the PDB file if you are willing.
The pdb file is just those atoms forming a tube where the iron atoms are touching each other to form the tube shape. But I am unable to upload the pdb file unfortunately.
So I opened the rtp for CHARMM 36, and I didn’t see an example of what an ion would look like in there. Where do you go to find out the format necessary for this file?