[gmx-users] How to use Buckingham potentials ?

[ramesh cheerla]

Dear Gromacs users,

                               I am planing  to use buckingham potential
for the non-bonded interactions  of my system. I know that by changing the
nbfunc to 2 in [ defaults ] directive of topology will allow to use the
Buckingham potential , But I don't know how to specify the  A, B,C values
in the ffnonbonded.itp file.
 what values should I specify in the  [ atomtypes ]  directive . As i know
only A,B,C values for the pairs of atoms  so i have tried by  specifying
A,B,C values both in  [ atomtypes ] and  [ pairtypes ] as

[ atomtypes ]
;name   at.num     mass          charge    ptype
A                      B                      C
CC32A      6       12.01100        0.1030    A       132277.5784
33.05785124    0.002710395
HCA2       1       1.00800         0.0355      A       29962.4608
41.58004158    0.000212547
OC30A      8       15.999400      -0.3480   A       243922.5976
40.2414486921  0.000803746
[ pairtypes ]
; i               j       func       A
B                        C
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


Doing so , grompp is showing the following errors

   ERROR 5 [file ffnonbondedpeo.itp, line 13]:
  Too many parameters or not enough parameters for topology B


ERROR 6 [file ffnonbondedpeo.itp, line 14]:
  Too many parameters or not enough parameters for topology B

Generated 6 of the 6 non-bonded parameter combinations
Excluding 3 bonded neighbours molecule type 'polymer'

ERROR 7 [file topol.top, line 1652]:
  ERROR: The cut-off length is longer than half the shortest box vector or
  longer than the smallest box diagonal element. Increase the box size or
  decrease rlist.
------------------------------------------------------
Program grompp, VERSION 4.5.5
Source code file: grompp.c, line: 1372

Fatal error:
There were 7 errors in input file
-------------------------------------------------------


Any help will be highly appreciated.


Regards,
Ramesh Cheerla
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