[gmx-users] filling missing param values

[gromacs query]

Dear All,

I am working on some molecule and I have defined atom types with OPLSaa and
all atom types really exist in oplsaa and well chosen so no approximation
in choosing atom types. During grompp, I found there are dihedral param
missing. e.g.
N CT CT NT
N CT CT N3

Is there any 'good' way of finding/assuming dihedral params for this? or I
should choose values of some nearby atom types?

Please suggest

regards,
Jiomm