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(May 16, 2021)

PIMC_NBEADSPERATOM

Number of path integral time slices (“beads”) used on each atom of a PIMC simulation.

TYPE:

INTEGER

DEFAULT:

None.

OPTIONS:

1
Perform classical Boltzmann sampling.
$>$1
Perform quantum-mechanical path integral sampling.

RECOMMENDATION:

This variable controls the inherent convergence of the path integral
simulation. The one-bead limit represents classical sampling and the
infinite-bead limit represents exact quantum-mechanical sampling. Using 32
beads is reasonably converged for room-temperature simulations of molecular
systems.

PIMC_TEMP

Temperature, in Kelvin (K), of path integral simulations.

TYPE:

INTEGER

DEFAULT:

None.

OPTIONS:

User-specified number of Kelvin for PIMC or classical MC simulations.

RECOMMENDATION:

None.

PIMC_MCMAX

Number of Monte Carlo steps to sample.

TYPE:

INTEGER

DEFAULT:

None.

OPTIONS:

User-specified number of steps to sample.

RECOMMENDATION:

This variable dictates the statistical convergence of MC/PIMC simulations.
For converged simulations at least ${10}^{5}$ steps is recommended.

PIMC_WARMUP_MCMAX

Number of Monte Carlo steps to sample during an equilibration period of MC/PIMC simulations.

TYPE:

INTEGER

DEFAULT:

None.

OPTIONS:

User-specified number of steps to sample.

RECOMMENDATION:

Use this variable to equilibrate the molecule/ring polymer before collecting
production statistics. Usually a short run of roughly 10% of
PIMC_MCMAX is sufficient.

PIMC_MOVETYPE

Selects the type of displacements used in MC/PIMC simulations.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0
Cartesian displacements of all beads, with occasional (1%) center-of-mass moves.
1
Normal-mode displacements of all modes, with occasional (1%) center-of-mass moves.
2
Levy flights without center-of-mass moves.

RECOMMENDATION:

Except for classical sampling (MC) or small bead-number quantum sampling
(PIMC), Levy flights should be used. For Cartesian and normal-mode moves,
the maximum displacement is adjusted during the warm-up run to the desired
acceptance rate (controlled by PIMC_ACCEPT_RATE). For Levy flights,
the acceptance is solely controlled by PIMC_SNIP_LENGTH.

PIMC_ACCEPT_RATE

Acceptance rate for MC/PIMC simulations when Cartesian or normal-mode
displacements are used.

TYPE:

INTEGER

DEFAULT:

None

OPTIONS:

$$
User-specified rate, given as a whole-number percentage.

RECOMMENDATION:

Choose acceptance rate to maximize sampling efficiency, which is typically
signified by the mean-square displacement (printed in the job output). Note
that the maximum displacement is adjusted during the warm-up run to achieve
roughly this acceptance rate.

PIMC_SNIP_LENGTH

Number of “beads” to use in the Levy flight movement of the ring polymer.

TYPE:

INTEGER

DEFAULT:

None

OPTIONS:

$3\le n\le \text{PIMC\_NBEADSPERATOM}$
User-specified length of snippet.

RECOMMENDATION:

Choose the snip length to maximize sampling efficiency. The efficiency can be
estimated by the mean-square displacement between configurations, printed at
the end of the output file. This efficiency will typically, however, be a
trade-off between the mean-square displacement (length of statistical
correlations) and the number of beads moved. Only the moved beads require
recomputing the potential, *i.e.*, a call to Q-Chem for the electronic energy.
(Note that the endpoints of the snippet remain fixed during a single move, so
$n-2$ beads are actually moved for a snip length of $n$. For 1 or 2 beads in
the simulation, Cartesian moves should be used instead.)

$comment The number of Monte Carlo steps is deliberately set low, more typical values would be: PIMC_WARMUP_MCMAX 10000 !Equilibration run PIMC_MCMAX 100000 !Production run $end $molecule 0 1 H H 1 0.75 $end $rem JOBTYPE pimc METHOD hf BASIS sto-3g PIMC_TEMP 298 PIMC_NBEADSPERATOM 32 PIMC_WARMUP_MCMAX 100 !Equilibration run PIMC_MCMAX 1000 !Production run PIMC_MOVETYPE 2 !Levy flights PIMC_SNIP_LENGTH 10 !Moves 8 beads per MC step (10-endpts) $end

$comment The number of Monte Carlo steps is deliberately set low, more typical values would be: PIMC_WARMUP_MCMAX 10000 !Equilibration run PIMC_MCMAX 100000 !Production run $end $molecule 0 1 H O 1 1.0 H 2 1.0 1 104.5 $end $rem JOBTYPE pimc METHOD rimp2 BASIS cc-pvdz AUX_BASIS rimp2-cc-pvdz PIMC_TEMP 500 PIMC_NBEADSPERATOM 1 !1 bead is classical sampling PIMC_WARMUP_MCMAX 100 !Equilibration run PIMC_MCMAX 1000 !Production run PIMC_MOVETYPE 0 !Cartesian displacements (ok for 1 bead) PIMC_ACCEPT_RATE 40 !During warm-up, adjusts step size to 40% acceptance $end