The lowest singlet-triplet excitation energy of BN: A converged coupled cluster perspective.

The notoriously small X 3Pi-a 1Sigma+ excitation energy of the BN diatomic has been calculated using high-order coupled cluster methods. Convergence has been established in both the one-particle basis set and the coupled cluster expansion. Explicit inclusion of connected quadruple excitations T4 is required for even semiquantitative agreement with the limit value, while connected quintuple excitations T5 still have an effect of about 60 cm(-1). Still higher excitations only account for about 10 cm(-1). Inclusion of inner-shell correlation further reduces Te by about 60 cm(-1) at the CCSDT, and 85 cm(-1) at the CCSDTQ level. Our best estimate, Te = 183+/-40 cm(-1), is in excellent agreement with earlier calculations and experiment, albeit with a smaller (and conservative) uncertainty. The dissociation energy of BN(X 3Pi) is De = 105.74+/-0.16 kcal/mol and D0 = 103.57+/-0.16 kcal/mol.