anchor.diff:  
           ! S found with the dichotomy approach
           sigma_t_thick(i) = dichotomy(k, i, optical_depth)
  +        ! sigma_t_thick(i) = secant(k, i, optical_depth)
   
           ! Optical thin case (tau < 1)
           optical_depth = 0
   
           ! S found with the dichotomy approach
           sigma_t_thin(i)  = dichotomy(k, i, optical_depth)
  +        ! sigma_t_thin(i) = secant(k, i, optical_depth)
  +
         enddo
   
         ! For the first critical point (the one at the right of the S shape)
  
positive.code: 
  
    !-------------------------------------------------------------------------
    ! secante method  in order to determine the change of sign in a given
    ! interval [Smin,Smax] with an epsilon precision
    !-------------------------------------------------------------------------
    real(x_precision) function secante(k, i, optical_depth)
      implicit none
  
      integer, intent(in) :: k ! Position in the disk, to get r/Omega
      integer, intent(in) :: i ! Iteration counter, to get the temperature
      integer, intent(in) :: optical_depth
      !-------------------------------------------------------------------------
      integer           :: j
      real(x_precision) :: T ! Temperature used for the secante
      real(x_precision) :: tau_eff
      real(x_precision) :: Smin     ! Lowest point
      real(x_precision) :: Smax     ! Highest point
      real(x_precision) :: f_min    ! Q+ − Q− at the lowest point
      real(x_precision) :: f_max    ! Q+ − Q− at the highest point
      !-------------------------------------------------------------------------
  
      T = temperature(i)
  
negative.code:     !------------------------------------------------------------------------
      integer                              :: i,k            ! Iteration counters
  
      ! For the variables
      real(x_precision)                    :: f              ! Q+ - Q-
      real(x_precision)                    :: tau_eff        ! Effective optical depth
      integer                              :: optical_depth  ! Indicator for the optical thickness
  
      ! For the separate curves given the thickness
      real(x_precision), dimension(nb_it)  :: sigma_t_thick
  