topology  
 RESIDUE SOD  ! Sodium Ion 
 
 GROUP  
       ATOM  SOD   TYPE=SOD  CHARGE=    1.0000  END  
 !END GROUP
 
 
 END {SOD }
 !-----------------------------------------------------------
 
 RESIDUE CL   ! Chloride Anion 
 
 GROUP  
       ATOM  CL1   TYPE=CLA  CHARGE=   -1.0000  END  
 !END GROUP
 
 
 END {CL  }
end

parameter
 NONBONDED  SOD      .1000   2.7297    .1000   2.7297 ! FROM BENOIT ROUX 
 NONBONDED  CLA      .1500   4.2763    .1500   4.2763 ! chloride 
end

segment                                                {*Generate ions     *}

   name="CL- "                              {*This name has to match the   *}
                                            {*four characters in columns 73*}
                                            {*through 76 in the coordinate *}
                                            {*file; in CNS this name is  *}
                                            {*referred to as SEGId.        *}
   chain
   coor
ATOM      1  CL1 CL      1       1.000   0.000   0.000  0.00  0.00      CL- 
   end
   end                                       {*obtain the sequence.        *}
end
                                             
coor
ATOM      1  CL1 CL      1       1.000   0.000   0.000  0.00  0.00      CL- 
end

segment                                                {*Generate ions     *}

   name="NA+ "                              {*This name has to match the   *}
                                            {*four characters in columns 73*}
                                            {*through 76 in the coordinate *}
                                            {*file; in CNS this name is  *}
                                            {*referred to as SEGId.        *}
   chain
   coor
ATOM      1  SOD SOD     1       0.000   0.000   0.000  1.00  0.00      NA+ 
   end
   end                                       {*obtain the sequence.        *}
end
                                             
coor
ATOM      1  SOD SOD     1       0.000   0.000   0.000  1.00  0.00      NA+ 
end

param

 NBONDS 

! -----NONBONDED-LIST-OPTIONS-------------------------------
  CUTNB=  13.000 TOLERANCE=    .000 WMIN=   1.500 ATOM   
  INHIBIT=    .250                                       
! -----ELECTROSTATIC OPTIONS--------------------------------
  EPS=   1.000 E14FAC=   1.000 CDIELECTRIC SHIFT SHFO BSHF=4.0
! -----VAN DER WAALS OPTIONS--------------------------------
  VSWITCH                                                
! -----SWITCHING /SHIFTING PARAMETERS-----------------------
  CTONNB=  10.000 CTOFNB=  12.000                        
! -----EXCLUSION LIST OPTIONS-------------------------------
  NBXMOD=   5                                            
! ----------------------------------------------------------

 END 

end

evaluate ($xx = 1.9)

while ($xx < 14.9) loop ANALYS

	evaluate ($xx = $xx + 0.1)
        do (x = $xx) (segid "CL- ")
        energy end
        show ave (dx) (segid "CL- ")
        eval ($fx = $result)
        display $xx	$ener	$fx	$elec	$vdw

end loop ANALYS

stop