```
subroutine check_start_rdm()
! This routine checks if we should start filling the RDMs -
! and does so if we should.
use rdm_general, only: realloc_SpawnedParts
use LoggingData, only: tReadRDMs, tTransitionRDMs
logical :: tFullVaryshift
tFullVaryShift = .false.
if (all(.not. tSinglePartPhase)) tFullVaryShift = .true.
! If we're reading in the RDMs we've already started accumulating them in a previous calculation
! We don't want to put in an arbitrary break now!
if (tReadRDMs) IterRDMonFly = 0
if (tFullVaryShift .and. ((Iter - maxval(VaryShiftIter)) == (IterRDMonFly + 1))) then
! IterRDMonFly is the number of iterations after the shift has changed that we want
! to fill the RDMs. If this many iterations have passed, start accumulating the RDMs!
IterRDMStart = Iter + PreviousCycles
IterRDM_HF = Iter + PreviousCycles
if (tEN2) tEN2Started = .true.
! We have reached the iteration where we want to start filling the RDM.
if (tExplicitAllRDM) then
! Explicitly calculating all connections - expensive...
! TODO: why are explicit RDMs not working with replicas?
if (tPairedReplicas) call stop_all('check_start_rdm', "Cannot yet do replica RDM sampling with explicit RDMs. &
& e.g Hacky bit in Gen_Hist_ExcDjs to make it compile.")
tFillingExplicRDMonFly = .true.
if (tHistSpawn) NHistEquilSteps = Iter
else
! If we are load balancing, this will disable the load balancer
! so we should do a last-gasp balance at this point.
if (tLoadBalanceBlocks .and. .not. tSemiStochastic) &
call adjust_load_balance(iter_data_fciqmc)
extract_bit_rep_avsign => extract_bit_rep_avsign_norm
! By default - we will do a stochastic calculation of the RDM.
tFillingStochRDMonFly = .true.
if (tTransitionRDMs) tTransitionRDMsStarted = .true.
call realloc_SpawnedParts()
! The SpawnedParts array now needs to carry both the spawned parts Dj, and also it's
! parent Di (and it's sign, Ci). - We deallocate it and reallocate it with the larger size.
! Don't need any of this if we're just doing HF_Ref_Explicit calculation.
! This is all done in the add_rdm_hfconnections routine.
end if
if (RDMExcitLevel == 1) then
write(stdout, '(A)') 'Calculating the 1 electron density matrix on the fly.'
else
write(stdout, '(A)') 'Calculating the 2 electron density matrix on the fly.'
end if
write(stdout, '(A,I10)') 'Beginning to fill the RDMs during iteration', Iter
end if
end subroutine check_start_rdm
```

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