[gmx-users] dihedral problem when converging Charmm FF into Gromacs format

[Baofu Qiao]

Dear all,

I meet a problem in understanding how to converge the Charmm FF into 
Gromat format for RNA system. In  ....../charmm27.ff/ffnabonded.itp 
(line 1084-1088), it says:
HN5    ON5     CN7B    CN7B    9       180.0   0.0      6
HN5    ON5     CN7B    CN7B    9       0.0     3.3472  3
HN5    ON5     CN7B    CN7B    9       180.0   0.0      2
HN5    ON5     CN7B    CN7B    9       180.0   0.0      1
HN5    ON5     CN7B    CN7      9       0.0     1.2552  3
HN5    ON5     CN7B    CN7      9       0.0     0.0        1

However, the original Charmm27 FF parameters are given as 
(Supporting_info in [Denning, Priyakumar, Nilsson & Macerell Jr., J. 
Comput. Chem. 2011, 32, 1929.])
Torsion               Vn/2 (c27)      Multiplicity   Phase
HN5   ON5   CN7B   CN7B   0.800   3   0.0
HN5   ON5   CN7B   CN7B   0.000   2   0.0
HN5   ON5   CN7B   CN7B   0.000   1   0.0
HN5   ON5   CN7B   CN7     0.300   3   0.0
HN5   ON5   CN7B   CN7     0.000   2   180.0
HN5   ON5   CN7B   CN7     0.000   1   0.0

Why there is a term with Multiplicity of 6 for HN5-ON5-CN7B-CN7B in 
Gromacs, but not in the original Charmm FF? and similarly a term of 
Multiplicity of 2 for HN5-ON5-CN7B-CN7 in the original Charmm FF, but 
not in Gromacs? Thanks a lot!

BTW: the Phase terms are also not equal to each other!

best wishes,
Baofu Qiao