[gmx-developers] Which function used for postion restraint?

[chicago.ecnu]

Dear Gromacs Developers,

    Harmonic potentials are used in Gromacs for imposing restraints on the motion of the system.
   
    V = 1/2 * k * (x-x0)^2
     
  And the forces are : F =  -k * (x-x0) . ...

   I want to use constant force for the restraint.  
  Just scale the force  : F = -k * (x-x0) / sqrt ( (x-x0)**2+(y-y0)**2 + (z-z0)**2 )  ...

  This was very simple.

  But I don't know where the position restraint code is . 

 Could you please tell me where I can start with ? 
 Which function or keyword I should search ?


 Many thanks for your help !


Best Wishes,
Yours Sincrely,
Chiango Ji