anchor.diff:  
   end type dmqmc_rdm_in_t
   
  -! [todo] - move to dmqmc_state_t
   ! Spawned lists for rdms.
   type rdm_spawn_t
       type(spawn_t) :: spawn
  
positive.code: 
  type dmqmc_rdm_in_t
  
      ! [todo] - rename.
      ! If true then the reduced density matricies will be calulated for the 'A'
      ! subsystems specified by the user.
  
negative.code:     ! calculated by reflecting spawning onto the lower triangle into the
      ! upper triangle. This is allowed because the density matrix is
      ! symmetric.
      logical :: half_density_matrix = .false.
  
      ! correlation_sites stores the site positions specified by the users
      ! initially (as orbital labels), to be used in the calculation of
      ! spin correlation functions.
      integer, allocatable :: correlation_sites(:)
      ! correlation_mask is a bit string with a 1 at positions i and j which
      ! are considered when finding the spin correlation function, C(r_{i,j}).
      ! All other bits are set to 0. i and j are chosen by the user initially.
      ! This is not actually an input option but is calculated from
      ! correlation_sites, but we store it here to be pragmatic.
      integer(i0), allocatable :: correlation_mask(:) ! (string_len)
  
      ! When using the old weighted importance sampling, dmqmc_sampling_probs
      ! stores the factors by which probabilities are to be reduced when spawning
  