[gmx-users] some puzzled about the dihedral paramaters

[Jinyao Wang]

gmx-users，
 Dear all,

  I make a *.itp for benzene by myself. All parameters come from the
ffoplsaabon.itp and ffoplsaanb.itp. while I have some puzzled about the
dihedral parameters of benzene. 
In ffoplsaabon.itp file, there are
dihedral parameters of benzene and the potential function is 3, like
this:
;X CA CA X 3 30.33400 0.00000 -30.33400 0.00000 0.00000 0.00000 ;
aromatic ring

but for improper dihedral of benzene there is some another dihedral
parameters of benzene and the potential function is 1,like this:
; Z -CA-X -Y improper torsion. CA is any ring carbon

#define improper_Z_CA_X_Y 1 180.0 4.60240 2

I think the improper dihedral can keep the molecule of benzene planar.
So I have no idea how to set dihedral the parameters of benzene. 

   I tryto make a BEN.itp like this, but I don't know if it is correct.
[ moleculetype ]
; Name            nrexcl
BEN             3

[ atoms ]
;   nr       type  resnr residue  atom   cgnr     charge       mass  
     1   CA     1    BEN      C1      1         -0.205
     2   CA      1    BEN      C2      2        -0.205 
     3   CA      1    BEN      C3      3         -0.205
     4   CA      1    BEN       C4     4         -0.205
     5   CA        1     BEN          C5      5         -0.205
     6   CA        1     BEN          C6      6         -0.205
     7   HA         1     BEN        H7     1          0.205
     8   HA      1    BEN       H8      2         0.205
     9   HA      1    BEN       H9      3         0.205
     10  HA    1    BEN      H10     4          0.205
     11  HA    1    BEN       H11     5          0.205
     12  HA    1    BEN      H12    6           0.205

[ bonds ]
;  i    j  func            length         force.c
    1     2     1     0.13942    392459.2
    1     6     1     0.13942    392459.2
    ...
    ...
[ angles ]
;  i    j    k  func          angle        force.c
    2     1     6     1    120.000    527.184
    2     1     7     1     120.000    292.880
   ...
   ...
[ dihedrals ]
 ; i     j      k     l   func      c0          c1     c2        c3      c4       c5
    6     1     2     3     3     30.33400    0.00000    -30.33400    0.00000    0.00000    0.00000    
    6     1     2     8     3     30.33400    0.00000    -30.33400    0.00000    0.00000    0.00000
    7     1     2     3     3     30.33400    0.00000    -30.33400    0.00000    0.00000    0.00000
    7     1     2     8     3     30.33400    0.00000    -30.33400    0.00000    0.00000    0.00000
    2     1     6     5     3     30.33400    0.00000    -30.33400    0.00000    0.00000    0.00000
    2     1     6    12     3     30.33400    0.00000    -30.33400    0.00000    0.00000    0.00000
   ...
   ...
[ dihedrals ]
;  ai    aj    ak    al funct            c0            c1            c2            c3            c4            c5
    7   1   2   6     1    180.0    4.60240        2
    8   2   3    1     1    180.0    4.60240        2
     9   3   2    4     1    180.0    4.60240        2
     10   4   3   5    1     180.0    4.60240    2
    11   5    4   6    1     180.0     4.60240    2
    12   6   5    1    1     180.0     4.60240    2
	


 				

　　　　　　　　Jinyao Wang
　　　　　　　　wangjy at ciac.jl.cn
　　　　　　　　　　2009-05-25