Calculations of 11 diatomic dissociation energies with coupled cluster theo
ry through iterative triple excitations highlight both the strength and lim
itations of this method. By combining very large basis sets (through septup
le zeta in some cases) and complete basis set extrapolations with correctio
ns for core/valence correlation, scalar relativistic and atomic/molecular s
pin-orbit effects, it was possible to achieve excellent agreement with expe
riment in most cases. However, for C-2 and CN the extent of the multiconfig
urational nature of the molecules caused problems for the single configurat
ion-based couple cluster methods. In the worse case, the inclusion of itera
tive triples resulted in a change with respect to the perturbative triples
result which was of the opposite sign to the full configuration interaction
change. This work emphasizes the difficulties in achieving uniform chemica
l accuracy even for ground state, first and second row diatomics. (C) 2000
American Institute of Physics. [S0021-9606(00)30626-2].