anchor.diff:    rho = PTread_i(config, 'rho')
     N = rho *L(1)*L(2)*L(3)
   
  +  T = PTread_d(config, 'T')
  +  d = PTread_d(config, 'd')
  +  
     tau = PTread_d(config, 'tau')
     N_MD_steps = PTread_i(config, 'N_MD')
     dt = tau / N_MD_steps
  
positive.code:   colloids% vel = 0
    
    call random_number(solvent% vel(:, :))
    solvent% vel = (solvent% vel - 0.5d0)*sqrt(6.d0*2.d0/3.d0)
    solvent% vel = solvent% vel - spread(sum(solvent% vel, dim=2)/solvent% Nmax, 2, solvent% Nmax)
    solvent% force = 0
    solvent% species = 1
  
    call solvent_cells%init(L, 1.d0)
    d = sigma_C + sigma_N + 0.5d0
    colloids% pos(:,1) = solvent_cells% edges/2.0
    colloids% pos(:,2) = solvent_cells% edges/2.0 
    colloids% pos(1,2) = colloids% pos(1,2) + d
  
negative.code:   double precision :: mass(2)
  
    double precision :: e1, e2
    double precision :: tau, dt , T
    double precision :: d
    double precision :: skin, co_max, so_max
    integer :: N_MD_steps, N_loop
  