GAS wave functions with connected spaces (to the limit of single inter-space excitations) the same result is obtained. The agreement between CAS and GAS wave functions with as little as single interspace excitations, n exc = 1, is impressive, even though the GAS space is considerably smaller than the CAS space. The CAS (12, 12) space consist of 853 776 SDs, while the GAS [4 • (3, 3)] space with single interspace excitations n exc = 1 consists of 468 942 SDs which is 55 % of the CAS space size. The largest energy difference between CAS and GAS with single interspace excitations, is obtained for the s = 0 spin state and is only 0.36 kJ mol -1 , a negligible quantity.