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?
I have never used OPLSaa before, but in principle every FF in GROMACS should be able to do that since the Phenyl ring is present in Phenylalanine. You could take a look at a topology of a molecule containing Phenylalanine and see how it is done there (applies to all FFs).
How many molecules do you have in your simulation box? Are the electrostatic interactions between the molecules very important for you? If yes - use GAFF and compute in advance the atomic charges with RESP routine (use at least three conformers).
I have found styrene topology made by D. van der Spoel et al, which helped me to realize that I made misteake in assigning oplsaa atom type for this questionable C atom (C3). Its type should be the same as for other C atoms in phenyl ring (opls_145, my setting was opls_141), which solved the problem.
Gromos FF might be suitable for polystyrene.
https://pdfs.semanticscholar.org/22d3/ce8aa85b35a872aabdaa330772d36b23a28d.pdf
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