# PCRITERIA

### From FEFF

* Advanced *

PCRITERIA pcritk pcrith

These criteria, like those described in the CRITERIA card, also limit the number of paths. However, they are applied in the pathfinder and eliminate unimportant paths while the pathfinder is doing its search. The pathfinder criteria do not know the degeneracy of a path and are therefore much less reliable than the curved wave and plane wave criteria in the CRITERIA card below. These path finder criteria (keep and heap) are turned off by default, and we recommend that they be used only with very large runs, and then with caution.

The keep-criterion pcritk looks at the amplitude of χ (in the plane wave approximation) for the current path and compares it to a single scattering path of the same effective length. To set this value, consider the maximum degeneracy you expect and divide your plane wave criterion by this number. For example, in fcc Cu, typical degeneracies are 196 for paths with large r, and the minimum degeneracy is 6. So a keep criterion of 0.08% is appropriate for a pw criteria of 2.5%.

The heap-criterion pcrith filters paths as the pathfinder puts all paths into a heap (a partially ordered data structure), then removes them in order of increasing total path length. Each path that is removed from the heap is modified and then considered again as part of the search algorithm. The heap filter is used to decide if a path has enough amplitude in it to be worth further consideration. If a path can be eliminated at this point, entire trees of derivative paths can be neglected, leading to enormous time savings. This test does not come into play until paths with at least 4 legs are being considered, so single scattering and triangular (2 and 3 legged) paths will always pass this test. Because only a small part of a path is used for this criterion, it is difficult to predict what appropriate values will be. To use this (it is only necessary if your heap is filling up, and if limiting rpath doesn’t help), study the results in ‘crit.dat’ from runs with shorter rpath and experiment with the heap criterion accordingly. In the future, we hope to improve this filter.

Before using these criteria, study the output in the file ‘crit.dat’ (use print option 1 for paths, see Table 2.1), which has the values of critpw, keep criterion, and heap criterion for all paths which pass the critpw filter.

Default: If this card is omitted, the keep and heap criteria are set to zero, that is, no filtering will be done at this step in the calculation.

* fcc Cu had degeneracies from 6 to 196, so correct for this by * dividing pw-crit of 2.5% by 30 to get 0.08 for keep crit. Check this * empirically by running with pcrits turned off and studying crit.dat. * After studying crit.dat, choose 0.5 for heap crit. PCRITERIA 0.08 0.5