28.6 FCIQMC output
The estimate of the energy is extracted from the simulation in various ways. Once in variable shift phase of the calculation, where the walker number
is stabilised (column 5 in output and FCIQMCStats file), the shift parameter (column 2) should vary about the correlation energy
( in Eq. 56) after equilibration.
Alternatively, a projected energy estimate can be obtained through the projection of the walker distribution onto a reference configuration
(column 9, 11 and 23 – see below for details), as
It is clear that this estimate is sensitive to the number of walkers on the reference configuration (), which is why the use of
the PROJECHANGEREF option is encouraged if it is unclear which the most highly weighted configuration is from the outset of
calculation. Initially, this reference will be chosen as the Hartree–Fock determinant, or the configuration resulting from the occupation
of the highest weighted orbitals in the case of a prior CASSCF/MCSCF calculation. Unless the system is very strongly correlated and no dominant
single reference exists, the projected energy estimate will usually have smaller errorbars, while at convergence the two values should provide
relatively independent energy estimates which should agree to within random errors.
The output file contains the following columns of data every INTERVAL iterations from the calculation, unless the NOMCOUTPUT option is given.
 Iteration number
 Current shift value
 Walker change from previous interval
 Walker growth rate
 Total walkers
 Total annihilated walkers over last interval
 Total walkers that died over last interval
 Total spawned walkers over last interval
 Averaged projected correlation energy estimate from beginning of calculation
 Averaged shift correlation energy estimate from 1000 iterations after entry into variable shift mode
 Instantaneous projected correlation energy estimate averaged only over last INTERVAL iterations
 Number of walkers currently residing on reference configuration
 Number of walkers currently residing on single or double excitations of reference configuration (that contribute to the sum in Eq. 58)
 Fraction of spawning attempts which are successful
 Total number of currently occupied configurations
 Time taken for last iteration
The end of the output also contains a summary of the most highly weighted configurations at the end of the simulation.
The FCIQMCStats file also contains this data, in a way that can easily be plotted, and also contains additional diagnostic information, detailed below
 Fraction of successful spawning events to single, rather than double excitations
 Range of occupied configuration number over between MPI processes – a measure of the parallel loadbalancing of the simulation
 Total imaginarytime which has elapsed in the calculation
 Ignore column
 Expected shift value obtained in fixed shift mode calculated from rate of growth of reference configuration
 Expected shift value obtained in fixed shift mode calculated from rate of growth of total walker number
 Projected total energy estimate averaged only over last INTERVAL iterations (col. 11 + reference energy)
 Denominator of projected energy estimate averaged only over last INTERVAL iterations
 Numerator of projected energy estimate averaged only over last INTERVAL iterations
 Instantaneous normalised weight of reference configuration in FCIQMC wavefunction
 Instantaneous normalisation factor of FCIQMC wavefunction
In addition to the FCIQMCStats file, unless the full scheme is being used, a file called INITIATORStats is created. Of interest in this
file are column 7: the number of `initiator' configurations, column 9: the number of walkers residing on `initiator' configurations and column 11: the
number of spawned walkers which were aborted due to the initiator approximation in the last INTERVAL iterations. A NodeFile file is also created
for each MPI process. Generally, these files will contain nothing that is not mirrored in the output file, however if the calculation terminates unexpectedly,
these files will often contain information on the processspecific reason for this, and should be checked in this instance. At the end of a calculation, the
output will print the highest weighted configurations in the calculation, as well as attempting an automatic error analysis on the final energies, which is discussed
in the next section.
Especially for initial investigations into a new system, it can be instructive to graphically view the progress of the simulation as it proceeds, to determine
whether the simulation can enter the variable shift phase, whether the population on the reference configuration is sufficient, or whether any other parameters
should be adjusted via the CHANGEVARS facility. Examples of typical plots starting from a single walker are shown in Fig. 1 and 2.
Figure 1:
Growth in total walker population (column 5), and population on the reference configuration (column 12), obtained from the working directory of the calculation
with the GNUPLOT package and the command set logscale y; plot 'FCIQMCStats' u 1:5 w l, ” u 1:12 w l.

Figure 2:
Instantaneous total energy estimate from column 23 of the FCIQMCStats file.

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