The accuracy of the energy-corrected sudden (ECS) formalism for line shape calculations is investigated, using coupled states calculation for CO2-Ar collisions on the recently developed "single repulsion" potential of Hutson er al. [J. Chem. Phys. 107, 1824 (1997); 105, 9130 (1996)]. Inelastic cross sections sigma(0)(L-->0,E) = Q(L)＇(E) are calculated using the MOLSCAT program, and then averaged over Maxwell-Boltzmann kinetic energy distributions to give the thermally averaged "basic rates" Q(L)＇(T) needed in the ECS formalism. The ECS linewidths for low initial J, J(i) less than or equal to 16, are sensitive only to the low-L basic rates, for which the CS calculations are converged; comparing them with directly calculated CS linewidths thus gives a stringent test of the ECS model, and it works well (within 10%). However, for higher J(i) lines and for band shape calculations, basic rates for higher L are needed for convergence. These are obtained by an extrapolation procedure based on experimental data, using an exponential power law and the adiabaticity factor recently suggested by Bonamy ct al. [J. Chem. Phys. 95, 3361 (1991)] ECS calculations using the resulting basic rates are designated "extrapolated CS-ECS calculations," and are found to give accurate results for high-J linewidths, for near-wing absorption and for band profiles over a very wide range of perturber pressures (up to 1000 arm).