[gmx-users] modifications required for the ffnonbonded.itp while adding a new residue

[ramesh cheerla]

Dear Gromacs users,


                           i am adding new residue to the existing charmm27
force field of the gromacs and i am planning to use buckingham potential
for the non bonded interactions. for that i made following changes to the
ffnonbonded.itp file

 [ atomtypes ]
;name   at.num  mass    charge  ptype   sigma   epsilon
CC32A      6       12.01100         0.1030  A       0.0
0.0            0.0
HCA2        1       1.00800          0.0355  A       0.0
0.0            0.0
OC30A      8       15.999400       -0.3480  A       0.0
0.0            0.0
 [ pairtypes ]
; i     j       func    sigma1-4        epsilon1-4 ; THESE ARE 1-4
INTERACTIONS
OC30A   OC30A   1         0.0  0.0
OC30A   CC32A   1         0.0  0.0
OC30A   HCA2     1         0.0  0.0
CC32A   CC32A    1        0.0  0.0
CC32A   HCA2      1        0.0  0.0
HCA2    HCA2       1        0.0  0.0
;########### I have given the A(in KJ/mol), B(in nm^-1), C(in Kj/mol*nm^6)
  [ nonbond_params ]
; i     j       func    sigma1-4        epsilon1-4 ; THESE ARE 1-4
INTERACTIONS
OC30A   OC30A    2       243922.5976    40.2414486921  0.000803746
OC30A   CC32A    2       179625.8144    36.297640653   0.001476115
OC30A   HCA2      2       85489.9984     40.899795501   0.000413379
CC32A   CC32A    2       132277.5784    33.05785124    0.002710395
CC32A   HCA2      2       62955.3928     36.832412523   0.000759396
HCA2    HCA2       2       29962.4608     41.58004158    0.000212547
;########### I have given the A(in KJ/mol), B(in nm^-1), C(in Kj/mol*nm^6)
;D = A, B = 1/P, C = E

and in the default directive of the forcefield.itp  i made the following
changes
 [ defaults ]
; nbfunc        comb-rule       gen-pairs       fudgeLJ fudgeQQ
2       1       no      0.0     0.0

what values should i give for the sigma and epsilon in the [ atomtype ]
directive and pairtypes directive . if i ignore the pair types it giving
error  no default Lj 1-4 interactions. though i am using nbfun as 2 to use
the  buckingham potential. I have only the values of the A,B,C. How to get
sigma and epsilon if they are required.
Did i modify the ffnonbonded.itp in the  correct manner ? or any thing went
wrong?

please help me in this regard.

Thank you in advance.

Regards,
Ramesh.
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