Although many options and details are provided here, often FCIQMC calculations can be performed using default parameters in a relatively `black-box' fashion, requiring knowledge of only a few key options.

The `FCIQMC` program is invoked by the command

`FCIQMC`,*OPTION=value*
*directives*

The simulation will then run within an orbital space obtained from the previous calculation (generally Restricted Hartree-Fock, although MCSCF/CASSCF/localized/unrestricted orbitals, as well as orbital subspaces can also be used with care). The electron number, spin-polarization and symmetry block of the FCIQMC calculation will also be presumed to be the same as specified for this preceding calculation, unless directives specify otherwise, and therefore should result in the FCIQMC module defining a `reference' configuration which is the same as the Hartree-Fock determinant in the previous calculation. The symmetry and energy of this determinant should be checked to avoid slow convergence, or one to an incorrect symmetry solution.

Directives are usually not necessary, but the `CORE`, `OCC`, `ORBITAL` and `WF` directives work as in
the MOLPRO CCSD program if core electrons are to be correlated, or if alternative orbital subsets or wavefunctions
wish to be defined contrary to the previous mean-field calculation.

The default running procedure is an -FCIQMC calculation,
initialized with a single walker at the reference determinant, and the shift parameter
fixed at zero. The number of initial walkers, or value for the shift can be adjusted from their default settings
with the `STARTWALKERS` and `FIXEDSHIFT` options. This initial stage should result in an
exponential growth of walkers, whose rate of growth
depends on the timestep, value of the fixed shift, and the correlation energy of the problem.
This phase of the calculation is called `fixed shift mode'. Once the total number of walkers has reached
a number given by `TARGETWALKERS`, the shift is updated every `INTERVAL` iterations,
in order to stabilize the walker growth.

The default behavior is to run for only 100 iterations. This is generally far too short, and should be changed using the `ITERATIONS` command,
the `TIME` command, or dynamically with the `CHANGEVARS` facility, which will be detailed in section 28.5.
In addition,
the timestep will be initially chosen as a reasonable upper-bound, and then dynamically updated
through the initial part of the calculation in order to
eliminate walker `blooming' events, where multiple walkers (of more than `MAXWALKERBLOOM` are
created in a single spawning attempt. However, this initial upper-bound can be a poor guess if the system is especially
small, and so alternatively, the timestep can be specified and fixed for the
entire calculation with the `TIMESTEP` option. Generally this wants to be as large as possible without causing frequent blooming events.

The available options are summarized in Table 12, and those requiring a more detailed explanation are expanded on in section 28.3.

Option | Default | Allowed values | Short description |

SYMMETRY |
ON | `ON/OFF' | Whether to use point-group symmetry |

SPIN |
OFF | `EVEN/ODD' | Whether to use spin-reversal symmetry |

ITERATIONS |
100 | INT | Maximum number of iterations |

TIME |
OFF | REAL | Runtime in minutes |

TIMESTEP |
N/A | REAL | Timestep for use in calculation |

STARTWALKERS |
1 | INT | Number of initial walkers on |

reference configuration | |||

TARGETWALKERS |
10,000 | INT | Target number of total walkers for |

entry to variable shift mode | |||

SEED |
-7 | INT | Random number seed |

STARTMP1 |
OFF | INT | Initialize value walkers at |

MP1 wavefunction distribution | |||

MAXATREF |
OFF | INT | Target number of walkers on reference |

configuration for variable shift mode | |||

FIXEDSHIFT |
0.0 | REAL | Initial value of the fixed shift |

SHIFTDAMPING |
0.1 | REAL | Damping parameter for adjustment of |

the shift in variable shift mode | |||

INTERVAL |
10 | INT | Iteration interval for writing out |

statistics and updating of the shift | |||

PROJE-CHANGEREF |
OFF | INT | Percentage increase from the current |

reference population required | |||

to change reference dynamically | |||

FULL_FCIQMC |
False | True/False | Flag to determine whether to run |

full FCIQMC or -FCIQMC algorithms | |||

INITIATOR_THRESH |
3 | INT | parameter for initiator criterion |

MAXWALKERBLOOM |
INIT_THSH |
INT | Maximum allowed spawning probability |

if automatically adjusting timestep | |||

TRUNCATE |
OFF | INT() | Truncate space by excitation level |

PARTIALFREEZEOCC |
OFF | INT | Number of lowest energy occupied |

spin-orbitals to partially freeze | |||

PARTIALFREEZEHOLES |
OFF | INT | Number of holes allowed in the |

PARTIALFREEZEOCC orbital space |
|||

PARTIALFREEZEVIRT |
OFF | INT | Number of highest energy virtual |

spin-orbitals to partially freeze | |||

PARTIALFREEZEVIRTELECS |
OFF | INT | Number of electrons allowed in the |

PARTIALFREEZEVIRT orbital space |

Option | Default | Allowed values | Short description |

MEMORYFACWALKERS |
1.5 | REAL | Memory factor for main walker array |

MEMORYFACSPAWNED |
0.3 | REAL | Memory factor for spawned walker arrays |

SINGLESBIAS |
OFF | REAL | Singles biasing factor in excitations |

READWALKERS |
False | True/False | Option to read in walker distribution |

from previous run (requires POPSFILEs) | |||

CONTINUEWALKERGROWTH |
False | True/False | If restarting, indicates whether to resume |

calculation in fixed/variable shift mode | |||

WRITEPOPSFILE |
OFF | INT | Write a restart file (POPSFILE) |

every value iterations |
|||

POPSFILETIMER |
OFF | REAL | Write a restart file (POPSFILE) |

every value hours |
|||

INCREMENTPOPSFILES |
False | True/False | If True preserve previous POPSFILEs |

REBLOCKSHIFT |
OFF | INT | Perform error analysis on data |

from previous calculation with | |||

specified iteration equilibration time | |||

for shift estimate | |||

REBLOCKPROJE |
OFF | INT | Perform error analysis on data |

from previous calculation with | |||

specified iteration equilibration time | |||

for projected energy estimate | |||

NOMCOUTPUT |
False | True/False | Suppress iteration-specific output |

(Note all data still written to | |||

FCIQMCStats file) |
|||

CALCMCSIZESPACE |
OFF | INT | Run routine to stochastically calculate |

size of symmetry-allowed FCI space |

molpro@molpro.net 2019-03-20