9.10.2 Job Control and Examples

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.)