Atomtype not found

GROMACS version: 2021.3
GROMACS modification: No

I am trying to simulate the diffusion of a small drug molecule across a lipid membrane.
The lipid membrane system was generated using CHARMM GUI. The input files provided by C-GUI were solvated in water and a NaCl concentration of 0.15 at a temperature of 310K. The membrane system was then minimized, equilibrated and produced using gromacs for 200ns.
In parallel, the input file for the small drug molecule was downloaded from RSCB PDB. Using avogadro, charmm-readable files of the drug were produced. Then CGenFF was used to to produce the topology.

To merge the two systems, gmx insert-molcules command was used as follows:
gmx insert-molecules -f prod.gro -ci md.gro -n index.ndx -o mem+drug.gro -replace -nmol 1 -seed 77
prod.gro is the equilibrated membrane system
md.gro is the equilibrate small drug molecule
index.ndx is the index file of the membrane system (used to specify that the drug molecule will be added to the solvent NOT the membrane)
mem+drug.gro is the new merged system

Following this step, the topology was updated to include the drug molecule as follows:
#include “dm21.itp” which is the .itp file for the drug was added after the forcefiled.itp
#include “” which is the topology file for the drug was added before the water.itp

Moreover, a new index file was generated to include the drug molecule in the SOLV index, making me have 2 index groups:
MEMB which contains all phospholipid groups & SOLV which contains Na+ ions, Cl- ions, water and the drug molecule.

While trying to minimize the system, I get the following error:
Atomtype CG2R61 not found
NB. This is the drug’s atomtype.

How do you think I can overcome this problem?
Thank you very much!

CHARMM-GUI files are meant to be used and not modified. If you include a new species, the force field files they provide will not have parameters related to it. Either create the system again entirely within CHARMM-GUi, or rebuild the topology of the system with our full CHARMM36 port (which includes the entire force field).

Thank you for your brief and straight-forward explanation! I was hoping I can add a new species to the membrane system after it had been fully equilibrated but I am seeing now that this is not possible. I will try either options and proceed accordingly. Thanks again!