In order to access this option you should select the EXPERT OPTIONS sheet (close to the upper left corner of the main program window).

The PCA option is designed mainly for specific *theoretical investigations*, whereas the UCA option should be used for comparison with *experimental data*.

Choose PCA, to select the **perturbative**
convolution approximation. For intermediate to high projectile velocities, the
corresponding results are very close to results from exact calculations within
1^{st} order perturbation theory.* Different forms or names for the
1^{st} order perturbation theory are the Bethe theory (the well-known
simple Bethe formula, is valid only at asymptotically high velocities and somewhat
less accurate than the PCA model), 1^{st} Born approximation, plane-wave
Born approximation (PWBA), and the so-called semi-classical approximation (SCA).
Note that only the latter yields directly impact-parameter dependencies (SCA
results are similar to the PCA results).

Choose UCA, to select the **unitary** convolution
approximation. The **UCA** model is appropriate for fast light ions and even
for fast *heavy ions.* Although some approximations are involved in the
UCA, the corresponding results seem to be more accurate for heavy ions than
exact classical-trajectory Monte Carlo results for S_e as well as for Q(b).
So far, only complex and time-consuming quantum calculations (AO coupled-channel
model and CDW-EIS model) seem to provide more accurate Q(b) results for fast
heavy ions on an ab initio basis, in comparison with the current code. For these
fast ions the accuracy of the current S_e results is comparable with some *fits
to experimental data*, as used, e.g., in the Ziegler-Biersack-Littmark code
TRIM or SRIM.

For intermediate to high projectile velocities, the corresponding integrated results S_e are very close to results from the Bloch theory, which accounts for the motion of electron wavepackets at high energy transfers.*