anchor.diff:    double precision :: g(3) !gravity
     logical :: fixed, on_track, stopped, order
     integer :: bufferlength
  +  integer :: steps_fixed
     fixed = .true.
     on_track = .true.
     stopped = .false.
  
positive.code:   N_MD_steps = PTread_i(config, 'N_MD')
    dt = tau / N_MD_steps
    N_loop = PTread_i(config, 'N_loop')
  
    
    sigma_C = PTread_d(config, 'sigma_C')
    sigma_N = PTread_d(config, 'sigma_N')
  
negative.code:      call dimer_io%velocity%append(colloids%vel)
       call dimer_io%image%append(colloids%image)
  
       if (i >= steps_fixed) then
          fixed = .false.
       end if
       
  