Probably you are using Gromacs. As you realize this error indicates that no parameters are available against the "LIG" residue in the applied force field. So you have to generate that. You can search in the literature to find out if any simulation has been carried out using that component in GROMACS so that you can asked them to send the parameter file. Otherwise you can use PRODRG server to generate the parameter.
Often people mimic the known one for example to generate the parameter of methyl cytosine you can use the parameter for cytosine with additional van der Waals torsional and charge parameter for methyl group. The scientific way is to do QM and generate the parameters.
It is better if someone already did the simulation with this "LIG" , since parameter development is tedious job and need rigorous validation.
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