  fsign = 1.d0
  t_val = time
  IF ( t_val .LT. 0.d0 ) THEN
    ! if t < 0 op%F(t,activeF) = -op%F(t+beta,activeF)
    t_val = t_val + op%beta
    fsign  = -1.d0 
  END IF

  ratioh = t_val * op%inv_dt + 1.d0 ! t / delta t
  index = INT(ratioh)
  ratioh = ratioh - DBLE(index)

  !  interpolation first order
  ! index + 1 can be sizeHybrid + 1 => value is 0 (cf. BathOperator_init routine)
  hybrid = fsign * (op%F(index,activeF) + &
      ratioh * ( op%F(index+1,activeF) - op%F(index,activeF) ))
