FORCE FIELD FILE IN NAMD

BONDS
!V(bond)=Kb(b-b0)**2   (This the contribution due to bonds as defined in equation 7,Module 3)
!Kb:kcal/mole
!bo:A
!atom type                   Kb                   b0
!
CA     N2                    350                  1.332   !                                                          
N2    CT                      300                  1.467   !
CT     H1                     350                  1.085   !          
N2     H                       490                  0.995   !          
C      H2                      376                  1.118   !          
C      O2                      650                  1.234   !

ANGLES
!
!V(angle)=Ktheta(Theta-Theta0)**2 (This the contribution due to angles as defined in equation 7,Module 3)
!
!Ktheta:Kcal/mole
!Theta0:degrees
!
!atom types                     Ktheta                     Theta0            
N2       CA      N2             74                          119                  !
CA      N2       H               50                          121.5               !
H         N2       H               26                          117                  !
CA      N2       CT             50                          121.9               !
CT       N2       CT             70                          115                  !
N2       CT       H1             52                          110                  !                      
H1       CT       H1             37                          108.9               !
H2       C         O2             80                          114.7               !
O2       C         O2             120                        130.5               !

DIHEDRALS
!
!V(dihedral)=Kchi(1+cos(n(chi)-delta))( (This the contribution due to dihedrals as defined in equation 7,Module 3)
!Kchi:Kcal/mol
!n:multiplicity
!delta:degrees
!
!atom types                             Kchi       n                   delta               

N2       CA      N2       H         1.8663    2                   180.00             !
N2       CA      N2       CT       2.7987    2                   180.00             !
CA      N2       CT       H1       0.000      3                   0.00                 !
CT       N2       CT       H2       0.1638     3                  0.00                 !
CT       N2       CT       H1       0.1638    3            0.00           

NONBONDED
!
!V(Lennard-Jones)=Eps,i,j[(Rmin,i,j/ri,j)**12-2(Rmin,i,j/ri,j)**6] (This portion consists of the fifth and sixth terms of equation 7,Module3)
!
!epsilon:Kcal/mol,Eps,i,j=sqrt(eps,i*eps,j)
!Rmin/2:A,Rmin,i,j=Rmin/2,i + Rmin/2,j
!Here Rmin/2 is the sigma value

!atom   ignored                        epsilon                         Rmin/2           

CA      0.0                               -0.08587                      1.700   !
N2       0.0                               -0.16976                      1.625   !
CT       0.0                               -0.10924                      1.700   !          
H1       0.0                               -0.01565                      1.236   !
H         0.0                               -0.01565                      0.535   !
O2       0.0                               -0.20969                      1.480   !
C         0.0                               -0.08587                      1.700   !
H2       0.0                               -0.01565                      1.1465 !

The definition of atom type and force field parameters can be found from

http://mackerell.umaryland.edu/CHARMM_ff_params.html

For e.g here CA represents the carbon atom within a imidazolium ring,while CT defines the carbon atom in the alkyl group.N2 refers to the nitrogen atom in the imidazolium ring.