Performance of CCSDT for diatomic dissociation energies

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].