[gmx-users] Dihedral angle and Fourirer Constant Help

[Pawan Babel]

Dear gmx-users,

Can anyone explain me why the order of the naming in ffoplsaabon.itp for
i,j ,k,l is different than usual convenction i.e. "i" is connected to "j"
and "j" is connected "k" and "k" is connected to "l" and thus (i-j-k-l)
represents a proper dihedral angle.

According to .../gromacs/top/ffoplaabon.itp

 [ dihedraltypes ]
;  i    j    k    l   func     coefficients
; OPLS Fourier dihedraltypes translated to Gromacs Ryckaert-Bellemans form
; according to the formula in the Gromacs manual.

 C_3    CT_2   N      C       3    -10.35749 -29.58716  -1.16734  41.11199
  0.00000   0.00000 ; C-term phi.

 C      CT     CT_2   NT      3      3.33465  -1.55226   2.82001  -4.60240
  0.00000   0.00000 ; H2N terminal

----------------
 I check the atom types from the topologies files and found out that
C_3 represents terminal CH3 carbon
CT_2 reperesent C-alpha carbon
C represent carbon attach to oxygen.

If i connect the atoms i-j-k-l in order then it does not form a proper
dihydral angle.

According to me, the order of the i-j-k-l should be either C_3 C N CT_2
 or CT_2 N C C_3 for C-terminal phi. So that it forms a proper
dihedral angle.

And similarly CT_2 C NT CT or CT NT C CT_2 for N-terminal.
If i don't change the order of the i-j-k-l in ffoplsaabon.itp i am getting
warning that there is " No default Ryckaret -bell types, using zeros".

Also please tell me how can i calculate fourier constants mention in
ffoplsaabon.itp.

Any help is higly appreciated.
Thanks in advance
Pawan Babel