Path types and rejection reasons

During a PyRETIS simulation, each Monte Carlo cycle generates (or loads) a path and records two short codes in the output:

• a move type (how the path was generated), and

• a status code (whether the path was accepted, and if not, why).

Both codes appear in the per-ensemble output files and in the simulation log, e.g.:

# Cycle: 42, status: ACC, move: sh

The tables below list every code that PyRETIS can produce.

See also

Adding a new move? Validate that it reproduces the reference rate with the reproducibility suite – see Extending and validating PyRETIS.

Move types

The move type describes how the new path was generated. For an explanation of what each move actually does – the basic moves, the sub-trajectory moves and infinite swapping – see Monte Carlo moves.

Code	Short name	Description
sh	Shoot	Path generated with a shooting move: a phase-space point is selected on the current path, momenta are perturbed, and new forward and backward trajectories are integrated. A path made by biased shooting is written sh as well; moves.txt records its attempt as bias.
ss	Stone skipping	Path generated by the Stone Skipping algorithm, a multi-step variant of shooting that reuses short sub-paths.
wt	Web Throwing	Path generated by the Web Throwing move.
wf	Wire Fencing	Path generated by the Wire Fencing move.
mw	Multiple wire fencing	Path generated by multiple wire fencing (mwf). The Mc column holds two characters, so pathensemble.txt writes the move as mw; moves.txt records it as mwf. The move shoots its segments from many points, so its O-shoot holds a marker value, not a shooting point.
ts	Target swap	Path generated by a target swap move.
tr	Time-reversal	Path generated by time-reversal: the current accepted path is reversed in time to produce the trial path.
s+	Swap from +	Path produced by the zero swap between [0^-] and [0^+] (the RETIS one, or the QuanTIS one when quantis = true), recorded in [0^-]: the incoming path came from the + (higher-index) ensemble.
s-	Swap from −	The same zero swap, recorded in [0^+]: the incoming path came from the - (lower-index) ensemble.
p+	PPTIS swap from +	Path produced by the PPTIS/REPPTIS swap between adjacent ensembles, recorded in the lower-index partner: the incoming path came from the + (higher-index) ensemble.
p-	PPTIS swap from −	The same PPTIS/REPPTIS swap, recorded in the higher-index partner: the incoming path came from the - (lower-index) ensemble.
mr	Mirror	Path generated by a mirror move.
hc	Held across rule change	A path an ensemble held at a restart that changed [tis] high_accept. It was accepted under the other acceptance rule, so, like an initialisation path, it is left out of the statistics until it is replaced.
ki	Kick initiation	Path generated by integrating the equations of motion after kicking (re-drawing) the initial momenta; used during path initiation.
is	Initial shoot	Path generated by a shooting move applied during the initial Stone Skipping preparation step, before the actual SS move.
ld	Load (unformatted)	Path loaded from unformatted (binary) external file(s).
rf	Load (formatted)	Path loaded from formatted (text) external file(s).
re	Restart	Path restored from a restart simulation. Unlike ki, ld and is, a re path (and an rf one) carries full, trusted phase points, so it is not treated as an initialisation path: the sampling continues normally and the path counts towards the production statistics. It contributes nothing to the generated-path-length distribution, though, because restoring it ran no dynamics.
00	Null move	Path produced by a null move: the current path is kept without any modification (used internally).

Path status codes

The status code explains why the trial path was accepted or, if rejected, what condition was violated.

Acceptance

Code	Meaning
ACC	The trial path was accepted and becomes the new current path.

Trajectory-length rejections

These rejections occur when the generated trajectory is too long or too short to satisfy the detailed-balance or max-length conditions.

Code	Meaning
BTL	Backward trajectory too long — the backward segment exceeded the limit imposed by the detailed-balance condition.
BTX	Backward trajectory too long — the backward segment exceeded the hard maxlength cut-off.
BTS	Backward trajectory too short — the backward segment ended before reaching a required interface.
FTL	Forward trajectory too long — the forward segment exceeded the limit imposed by the detailed-balance condition.
FTX	Forward trajectory too long — the forward segment exceeded the hard maxlength cut-off.
FTS	Forward trajectory too short — the forward segment ended before reaching a required interface.
XSS	Stone Skipping sub-path too long — an internal sub-path within the Stone Skipping algorithm exceeded the allowed length.

Interface / crossing rejections

Code	Meaning
NCR	No crossing with the middle interface — the trial path did not cross the required central interface of the ensemble.
BWI	Backward trajectory ends at wrong interface — the backward segment terminated at an interface other than the one required.
EWI	Initial path ends at wrong interface — used during path loading/initiation: the loaded path ends at an interface that is not compatible with the ensemble.
SWI	Initial path starts at wrong interface — used during path loading/initiation: the loaded path starts at an interface that is not compatible with the ensemble.
0-L	{0−} path ends at left interface — for paths in the {0-} ensemble the path ended at the left (A-state) interface instead of crossing to the right.
NSS	No one-step crossing in Stone Skipping — the Stone Skipping algorithm could not find a valid one-step interface crossing.

Momentum / energy rejections

Code	Meaning
MCR	Momenta change rejection — the new momenta drawn during the shooting move were rejected (e.g. by the Metropolis criterion on the kinetic energy).
KOB	Shooting point outside of boundaries — the shooting point lies below the left interface or above the right interface of the ensemble. Both propagation legs start from the shooting point and would stop on it, so the trial is rejected before any propagation. The point tested is the one after the momentum kick, which moves a velocity-dependent order parameter, or the crossing frame that the initial shooting move (is) of Stone Skipping shoots from, which in the {0+} ensemble lies below the left interface for one frame of every crossing.

Detailed-balance / super-detailed-balance rejections

These rejections enforce the (super) detailed-balance conditions for advanced moves.

Code	Meaning
HAS	High-acceptance swap rejection — the high-acceptance criterion for Stone Skipping / Wire Fencing detailed balance was not satisfied.
SSA	Stone Skipping super-detailed-balance rejection — the Stone Skipping move was rejected by the super-detailed-balance acceptance test.
WFA	Wire Fencing super-detailed-balance rejection — the Wire Fencing move was rejected by the super-detailed-balance acceptance test.
WTA	Web Throwing super-detailed-balance rejection — the Web Throwing move was rejected by the super-detailed-balance acceptance test.
SAR	Selection-bias acceptance rejection — a valid biased shooting trial path was rejected by the selection-bias Metropolis test \(\min(1, W_\mathrm{old}/W_\mathrm{new})\).

Target-swap rejections

Code	Meaning
TSS	Target swap selection rejection — the target-swap move was rejected at the selection step (no suitable target ensemble was found).
TSA	Target swap detailed-balance rejection — the target-swap trial was rejected by the detailed-balance acceptance criterion.

REPPTIS-specific rejections

Code	Meaning
SWD	REPPTIS swap — wrong propagation direction — in a REPPTIS swap move the trial trajectory propagated in the wrong direction relative to the interface.
SWH	REPPTIS swap — first-half extension rejected — the first half of the REPPTIS swap extension was rejected.

Other

Code	Meaning
NSG	No suitable segments — the path contains no segments that can be used as shooting points.
EXP	Exploration path — the path was generated during an exploration phase and is not counted as a regular MC step.