Full Configuration Interaction Quantum Monte Carlo¶

fciqmc {
    sys = system,
    qmc = { ... },
    fciqmc = { ... },
    semi_stoch = { ... },
    restart = { ... },
    reference = { ... },
    load_bal = { ... },
    logging = { ... },
    output = { ... },
    blocking = { ... },
    qmc_state = qmc_state,
}

Returns:: a qmc_state object.

fciqmc performs a full configuration interaction quantum Monte Carlo (FCIQMC) calculation [Booth09] on a system.

Options¶

sys

type: system object.

Required.

The system on which to perform the calculation. Must be created via a system function.

qmc

type: lua table.

Required.

Further options that are common to all implemented QMC algorithms. See qmc options.

fciqmc

type: lua table.

Optional.

Further options to control the FCIQMC algorithm. See fciqmc options.

semi_stoch

type: lua table.

Optional.

Further options to control using a semi-stochastic projection of the Hamiltonian operator instead of a purely stochastic projection. Note that some options in the semi_stoch table are required to be set if the table is given. See semi_stoch options.

restart

type: lua table.

Optional.

Further options to control restarting the calculation from a previous calculation. See restart options.

reference

type: lua table.

Optional.

Further options to select the reference state used. See reference options.

load_bal

type: lua table.

Optional.

Further options to improve the parallel load balancing of an FCIQMC simulation. If present (even if empty) an advanced load-balancing algorithm is used [Malone16a]. See load_bal options for more details.

logging

type: lua table.

Optional.

Further options to enable various logging outputs from a FCIQMC simulation. Only available when compiled in debug mode. See logging options for information on current options.

output

type: lua_table.

Optional.

Further options to enable direction of calculation output to a different file. See output options for more information.

blocking

type: lua table.

Optional.

Further options to switch on and control blocking on the fly. See blocking options.

qmc_state

type: qmc_state object.

Optional.

Output of a previous calculation to resume.

Warning

The qmc_state object must have been returned by a previous FCIQMC calculation. The validity of this is not checked. The system must also be unchanged.

Warning

This destroys the qmc_state object and so it cannot be re-used in subsequent QMC calculations.

fciqmc options¶

select_reference_det

type: boolean or Lua table.

Optional. Default: false.

If true or if a lua table is provided, attempt to automatically set the reference state to be the state with the greatest population. A lua table can contain the following options and need only be provided in order to modify the defaults.

Note

Care should be take when analysing the projected estimator to ensure that all quantities averaged have the same reference state.

Warning

Excitation levels are relative to the reference state and hence this should not be used with a truncated CI calculation.

update_every

type: integer

Optional. Default: 20.

The number of report loops between attempts to update the reference state.

pop_factor

type: float.

Optional. Default: 1.5.

The factor of the reference population another state must have in order for the reference to be changed. This helps prevent continually switching between states with similar or degenerate populations.

non_blocking_comm

type: boolean.

Optional. Default: false.

Use non-blocking MPI communications instead of blocking MPI communications.

Note

This is an experimental option and may or may not improve performance. In particular, its efficiency is highly dependent upon architecture and MPI implementation. For expert use only!

load_balancing

type: boolean.

Optional. Default: false.

Enable dynamic load balancing of determinants among processors. This will move determinants to try and keep the number of walkers on each processor roughly constant. See load_bal options for more details.

init_spin_inverse_reference_det

type: boolean.

Optional. Default: false.

In addition to initialising the reference determinant with an initial population, initialise the spin-inversed determinant (if different) with the same population. Overridden by a restart file.

trial_function

type: string.

Optional. Default: ‘single_basis’.

Possible values: ‘single_basis’, ‘neel_singlet’ (Heisenberg model only).

The trial function to use in the projected energy estimator. ‘single_basis’ uses the single reference state as the trial function. ‘neel_singlet’ uses the Neel singlet state, \(|NS \rangle = \sum_{i} a_i |D_i \rangle\), where the amplitudes \(a_i\) are defined in K. Runge, Phys. Rev. B 45, 7229 (1992).

Using a multi-reference trial function can substantially reduce stochastic noise.

guiding_function

type: string.

Optional. Default: ‘none’.

Possible values: ‘none’, ‘neel_singlet’ (Heisenberg model only).

The importance sampling transformation to apply to the Hamiltonian.

‘neel_singlet’ uses the Neel singlet state (K. Runge, Phys. Rev. B 45, 7229 (1992)) to transform the Hamiltonian such that the matrix elements, \(H_{ij}\), are replaced with \(a_i H_{ij} / a_j\). Using ‘neel_singlet’ automatically sets trial_function to ‘neel_singlet’.

replica_tricks

type: boolean.

Optional. Default: false.

Perform replica simulations (i.e. evolve two independent FCIQMC simulations concurrently).

density_matrices

type: boolean.

Optional. Default: false.

Samples the 2-RDM and reports the trace and numerator of the energy estimate. Prints the Hermitian part of the 2-RDM once the calculation has completed.

Requires replica_tricks to be enabled.

Note

Both replicas must be in variable shift mode in order for the 2-RDM to be calculated.

density_matrix_file

type: string.

Optional. Default: RDM

The name of the output file that the final 2-RDM is printed to. The (normalised, Hermitian part of) the 2-RDM is given in physical notation.

density_matrix_report

type: int.

Optional. Default: 3000

The minimum report cycle the 2-RDM statistics are collected after.

load_bal options¶

The default values are usually sufficient if load balancing is enabled. It is highly recommended to only attempt to improve load balancing for large calculations and once the population has been stabilised by the shift. It may be easiest to do this by monitoring a calculation carefully until this condition is reached, producing a restart file and then running a production calculation with load balancing enabled.

nslots

type: integer.

Optional. Default: 20.

The average number of slots per processor used to distribute the list of occupied states via a hashing of the states. A large value will affect performance but could potentially result in a better distribution of walkers.

min_pop

type: integer.

Optional. Default: 1000.

The minimum total population required before load balancing is attempted. This is a system dependent value and, in order to maximise performance improvements, should be set such that the population is roughly stable.

target

type: float.

Optional. Default: 0.05.

Desired imbalance (as a percentage of the average population per processor) between the most and least populated processors. Note that the workload on a processor is not entirely determined by its population and that, due to the algorithms used, an arbitrary small population imbalance is not usually possible.

max_attempts

type: integer.

Optional. Default: 2.

The number of attempts to make to improve load balancing. Often multiple attempts can improve the balancing but each attempt may be non-negligible and there are usually diminishing returns.

write

type: boolean.

Optional. Default: false.

Write out the population of the most and least heavily populated processor before and after load balancing is carried out. Also print out the minimum slot population on the most populated processor which will indicate if load balancing is possible.