Abstract The ground state potential curve of Cl2 has been computed near Rc by means of the SCF, MC SCF, CI(SD), and the recently proposed CPF methods. The convergence of the total energy, of Dc and Rc is studied with the aid of computations for various basis sets which include up to three d, two f and one g set. Higher polarization functions have a larger effect than for F2 for all methods, the g set still affects Dc by 0.15 eV and Rc by 0.02 a0 on the CPF level. The most elaborate calculation, on the CPF [7,4,3,2,1] level, yields Dc and Rc with an accuracy of 0.08 eV and 0.02 a0. The same accuracy is obtained for the MC-178 CAS SCF treatment employing a 2d1f polarization basis set. The present results allow us to order the polarization functions according to their relative importance as: d(1) > f(1) > d(2) > f(2) = d(3) > g(1) for the SCF and d(1) > f(1) > d(2) > g(1) > f(2) = d(3) for methods including external correlation. CI(SD) or CPF. A comparison of the results for N2, F2, P2, and Cl2 shows the higher polarization functions d(3), f(2), g(1) to contribute 0.1 (F2) to 0.3 eV (F2, Cl2) to Dc and affect Rc by 0.005 a0 (N2) to 0.01 7 a0 (P2). We have focused our attention mainly on the impact of the atomic basis set incompleteness on the accuracy of results obtained from various methods of computation. Cl2 turns out to be more demanding than the first-row counterpart F2 on all levels of theory, which is mainly due to the slower angular convergence rate for E, Dc and Rc (for Cl2), tables 2 and 3. The more pronounced influence of higher polarization functions is already noticed on the SCF and MC SCF level: f(1) increases Dc by 0.18 eV and reduces Rc by 0.052 a0 on the MC-178 level for Cl2, table 6, typical corresponding results for F2 are only 0.04 eV and 0.013 a0 [1]. CAS SCF calculations furthermore appear to require larger active spaces for Cl2, as discussed in section 3.3. On the CI(SD) or CPF level — which aim to account for the entire external correlation — one even finds a pronounced influence of the first g set which contributes ≈ 0.15 eV to Dc and reduces Rc by ≈ 0.02 a0 (on the CPF level, table 3), the corresponding effects for F2 were only ≈ 0.04 eV and 0.01 a0 [1]. The 2d1f polarization basis, which will remain the “standard” large basis for treatments of tri- and tetraatomic molecules, appears to underestimate Dc by still 0.5 eV and to overestimate Rc by ≈ 0.02 a0 for P2 and Cl2, table 7, and probably all molecules in-between. This conclusion emerges from the cumulative effect of adding d(3), f(2) and g(1) which amounts already to 0.3 eV and 0.015 a0, table 7.