Dear Gromacs experts,

I'm dealing with the box randomly packed with OLED material. For MM simulations in Gromacs, I use the FF recently suggested for a series of similar molecules.

As the next step, I have included an emitter molecule (E) in the center of preequiliblrated box and proceeded with QM/MM calculations. As the mixing of different FFs is not recommended to use, I do not need to have an appropriate FF for E. But even if I want my E to be frozen in the center of box, I must have a topology file for E, and, most importantly, "sigma" and "epsilon" parameters of LJ-potential for atom types. Without defining LJ, the atoms of QM (or frozen!) and MM parts do not feel each other and one naturally get unphysical overlap of subsystems.

To solve the problem, I have tried to generate a FF for E molecule via ATB site and then I have used topology file and LJ parameters from there. But I'm not sure this is an optimal way of treating the problem...

If I want to study some other molecule (E1) embedded in the OLED box, should I really first generate a FF which is not needed as E1 will be treated as QM? My emitters are not included in some other known force fields.

I would appreciate your advices.