subroutine generate_fci_space
  use bitmasks
  implicit none
  BEGIN_DOC
! Generates the complete FCI space
  END_DOC
  integer :: i, sze, ncore
  integer(bit_kind) :: o(N_int,2)
  integer(bit_kind) :: u, coremask

  if (mo_num > 64) then
     stop 'No more than 64 MOs'
  endif

  ncore = 0
  coremask = 0_bit_kind
  do i=1,mo_num
    if (trim(mo_class(i)) == 'Core') then
      ncore += 1
      coremask = ibset(coremask,i-1)
    endif
  enddo
  o(1,1) = iand(full_ijkl_bitmask(1),not(coremask))
  o(1,2) = 0_bit_kind

  call configuration_to_dets_size(o,n_det_alpha_unique,elec_alpha_num-ncore,N_int)
  TOUCH n_det_alpha_unique

  integer :: k,n,m, t, t1, t2
  k=0
  n = elec_alpha_num
  m = mo_num - n
  n = n

  u = shiftl(1_bit_kind,n) -1
  do while (u < shiftl(1_bit_kind,n+m))
    if (iand(coremask, u) == coremask) then
      k = k+1
      psi_det_alpha_unique(1,k) = u
    endif
    t = ior(u,u-1)
    t1 = t+1
IRP_IF WITHOUT_TRAILZ
    t2 = shiftr((iand(not(t),t1)-1), popcnt(ieor(u,u-1)))
IRP_ELSE
    t2 = shiftr((iand(not(t),t1)-1), trailz(u)+1)
IRP_ENDIF
    u = ior(t1,t2)
  enddo


  call configuration_to_dets_size(o,n_det_beta_unique,elec_beta_num-ncore,N_int)
  TOUCH n_det_beta_unique

  k=0
  n = elec_beta_num
  m = mo_num - n
  u = shiftl(1_bit_kind,n) -1
  do while (u < shiftl(1_bit_kind,n+m))
    if (iand(coremask, u) == coremask) then
      k = k+1
      psi_det_beta_unique(1,k) = u
    endif
    t = ior(u,u-1)
    t1 = t+1
IRP_IF WITHOUT_TRAILZ
    t2 = shiftr((iand(not(t),t1)-1), popcnt(ieor(u,u-1)))
IRP_ELSE
    t2 = shiftr((iand(not(t),t1)-1), trailz(u)+1)
IRP_ENDIF
    u = ior(t1,t2)
  enddo

  call generate_all_alpha_beta_det_products

  print *, N_det


end