anchor.diff:   
         Return
         END SUBROUTINE
  +
  
positive.code:                R_square_i = 1.0/R_square
                 sigma_square = sigma**2.0
                 R_six_i = (R_square_i*sigma_square)**3.0
                 
                 Upot  = Upot + 4.00*eps*R_six_i*(R_six_i - 1.00) - Ecut
                 Ff    = 48.0*eps*R_square_i*R_six_i*(R_six_i - 0.5)
                 Press = Press + Ff
                 Ff    = Ff*R_square_i
  !  Update the total force on atoms
  
negative.code:          DO J = I + 1,Natom
   
  ! Calculate the distance between the two atoms
              Dx = Xx(I) - Xx(J)
              Dy = Yy(I) - Yy(J)
   
  ! Apply the periodic boundary conditions
              Dx = Dx - Box*nint(Dx/Box)
              Dy = Dy - Box*nint(Dy/Box)
   
              R_square = Dx*Dx + Dy*Dy  
  ! Determine the Lennard-Jones parameters dependent on the two atom types          
               Type1 = AT(I)   !Array list of atom type
               Type2 = AT(J)  
              
               ! Type 1 and 2 will be 1-4 integer of atom type
               Rcut = R_cut_matrix(Type1,Type2)
  