THE COMPUTATION OF C-C AND N-N BOND-DISSOCIATION ENERGIES FOR SINGLY, DOUBLY, AND TRIPLY BONDED SYSTEMS

The bond dissociation energies (D(e)) of C2H2, C2H4, C2H6, N2, N2H2, and N2H4 are studied at various levels of correlation treatment. The convergence of D(e) with respect to the one-particle basis is studied at the single-reference modified coupled-pair-functional (MCPF) level. At all levels of correlation treatment, the errors in the bond dissociation energies increase with the degree of multiple bond character. The multireference configuration-interaction (MRCI) D(e) values, corrected for an estimate of higher excitations, are in excellent agreement with those determined using the size-extensive averaged-coupled-pair-functional (ACPF) method. We find that the full-valence complete-active-space self-consistent-field (CASSCF)/MRCI calculations are reproduced very well by MRCI calculations based on a CASSCF calculation that includes in the active space only those electrons involved in the C-C or N-N bonds. To achieve chemical accuracy (1 kcal/mol) for the D(e) values of the doubly bonded species C2H4 and N2H2 requires one-particle basis sets including up through h angular momentum functions (l = 5) and a multireference treatment of electron correlation: still higher levels of calculation are required to achieve chemical accuracy for the triply bonded species C2H2 and N2.