[gmx-users] force constants of improper dihedrals in .itp file from prodrg

[Joaquim Rui Rodrigues]

Dear all,

I am checking the itp file for my ligand obtained from prodrg beta, using gromos96.1 ff (43a1).  In the 
[ dihedrals ] section, I found lines such as 

; ai  aj  ak  al  fu    c0, c1, m, ...
  31  32  24  28   2     35.3  334.8       35.3  334.8   ; imp    C4   N1   O1   C2   
  32  31  33  37   2      0.0  167.4        0.0  167.4   ; imp    N1   C4  C12  C11   

for tetrahedral centres and substituents of rings, respectively,  that make sense, but also

  32  33  35  36   2      0.0  209.3        0.0  209.3   ; imp    N1  C12   N2  C13  

for out-of-plane bending, for which I don't see where the force value (209.3) come from. From the 
ffG43a1bon.itp file, there are only 3 improper types:

#define gi_1           0.0   167.42309
; planar groups 40      
;
#define gi_2      35.26439   334.84617
; tetrahedral centres   80      
;
#define gi_3           0.0   669.69235
; heme iron     160     

Can someone explain how prodrg get the value for the force constant for out-of-plane bending of rings?
Also,  what is meaning of the (commented) values 40, 80 and 160 in this excerpt of ffG43a1bon.itp?

Many thanks,
Rui Rodrigues


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