I was trying Oplsaa, which worked fine for me for polyethelene. But in the case of PS I had problems because grompp produced many warnings the same type: "No default Angle types, using zeroes". These errors concern the topology file, the part with 3-body angle. The problem is not connected with all atoms, but only one specific C atom in each styrene monomer: the one that connects the backbone with the phenyl ring. Each styrene part produces 3 warnings like above for C4-C3-C8, C3-C4-H and C3-C8-H interaction. I did not notice any other warnings.
Below I made bold the parts which produced warnings
C1-C2-
|
C3
// \
H-C4 C8-H
| ||
C5 C7
\\ /
C6
Can I then use oplsaa for PS in Gromacs?