This proposed Workshop is intended to compare the theoretical approaches, methodologies and computational implementations for the calculation of ion-atom collision cross sections. A large number of experimental results are available for comparison in the case of one-electron or quasi-one-electron systems in their ground state, and agreement with theory is generally good (though there are some well-known discrepencies, for example at around 50 keV for the simple H^{+} + H(1s) system).

However, for systems in which the target atom is initially in an excited state, which are of interest in fusion plasma modelling and diagnostics, very few *ab initio* calculations and virtually no experimental data exist. These systems are characterized by very large cross sections but require the consideration of a very large number of states (and angular momenta) to treat accurately.

Motivated by the present lack of data, the proposed Workshop will focus on a single test system, Be^{4+} + H, tackled using the following independent methods:

- SC MO close-coupling
- Classical CTMC
- Semiclassical BGM close-coupling
- CCC
- SC AO close-coupling

Three projectile energies: 20, 100 and 500 keV/u are to be considered; these are chosen to be of relevance to fusion plasmas and overlap the typical validity range of the selected theoretical methods. The hydrogen target is considered in the ground state (1s) and excited states 2s, 2p_{m} (*m* = 0, ±1).

The aim is to produce and compare data for a collision system in order to evaluate and assess these methods and to estimate the typical accuracy of the calculated cross sections for:

- Total electron capture
- Total ionization
- Total excitation
- State-selective processes (for the dominant channels).

The calculations will be performed in the second half of 2019 for comparison in early 2020.

This Workshop was conceived in order to support the aims of the Neutral Beams CRP which will provide evaluated and recommended data for the principal atomic processes relevant to heating and diagnostic neutral beams in fusion plasmas.

6 participants from 5 countries.