[gmx-users] Purpose of repeated improper dihedrals

[Dawid das]

Dear All,

Why some of the improper parameters in CHARMM27 FF are repeated with
the middle atom types in a different order as in, e.g.

HR1     NR1     NR2     CPH2    2       0.0000  4.184
HR1     NR2     NR1     CPH2    2       0.0000  4.184

while some are not, e.g.
HR3     CPH1    NR3     CPH1    2       0.0000  8.368

Best regards,
Dawid Grabarek