anchor.diff:          ! Don't allow creation of anti-particles in simple_fciqmc.
           if (nkill > abs(walker_population(1,iwalker))) then
               write (6,*) iwalker, walker_population(1,iwalker), &
  -            abs(walker_population(1,iwalker))*tau*(hamil(iwalker,iwalker)-shift)
  +            abs(walker_population(1,iwalker))*tau*(hamil(iwalker,iwalker)-H00-shift)
               call stop_all('do_simple_fciqmc','Trying to create anti-particles.')
           end if
   
  
positive.code:         ! We store the Hamiltonian matrix rather than the K matrix.
          ! It is efficient to allow all particles on a given determinant to
          ! attempt to die in one go (like lemmings) in a stochastic process.
          rate = abs(walker_population(1,iwalker))*tau*(hamil(iwalker,iwalker)-shift)
          ! Number to definitely kill.
          nkill = int(rate)
          rate = rate - nkill
  
negative.code:         if (nprocs > 1) call stop_all('init_simple_fciqmc','Not a parallel algorithm.')
  
          ! Find and set information about the space.
          call set_spin_polarisation(ms_in)
          call find_sym_space_size()
  
  