In order to access this option you should select the EXPERT OPTIONS sheet (close to the upper left corner of the main program window).
At lower projectile velocities the original UCA and PCA predictions become false, since for all shells the initial orbital velocity is replaced by zero (this results from a peaking approximation inside the UCA and PCA models). This uncertainty, however, affects only the close collision treatment and not the long-ranged dipolar interactions. Since close encounters are well described by classical two-body collisions, we make use of the kinetic theory by Sigmund (based on the classical binary-encounter model). [P. Sigmund, A. Schinner; NIM-B 195, 64 (2002)] Hence, the close collision fraction of the stopping cross section
2 Z^2 /b^2 h(2 v b) /v^2
is replaced by
2 Z^2 /b^2 <h(2 (vp-v2) b) /(vp-v2)^2>,
where <…> denotes a 3D-average over a hydrogenic momentum distribution calculated for an effective target charge determined from the first excitation energy.
When selecting this option the range of validity of the UCA results is extended by about an order of magnitude down in energy (below 20 keV/u instead of about 150 keV/u without shell correction). This option affects stopping due to target ionization as well as due to projectile-electron loss.
Since CasP 5.0 version this option uses renormalized shell corrections to fit the exact shell corrections.