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

The notoriously small XΠ 3 −aΣ + 1 XΠ3−aΣ+1 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 T ˆ 4 T̂4 is required for even semiquantitative agreement with the limit value, while connected quintuple excitations T ˆ 5 T̂5 still have an effect of about 60cm −1 60cm−1 . Still higher excitations only account for about 10cm −1 10cm−1 . Inclusion of inner-shell correlation further reduces T e Te by about 60cm −1 60cm−1 at the CCSDT, and 85cm −1 85cm−1 at the CCSDTQ level. Our best estimate, T e =183±40cm −1 Te=183±40cm−1 , is in excellent agreement with earlier calculations and experiment, albeit with a smaller (and conservative) uncertainty. The dissociation energy of BN(XΠ 3 ) BN(XΠ3) is D e =105.74±0.16kcal∕mol De=105.74±0.16kcal∕mol and D 0 =103.57±0.16kcal∕mol D0=103.57±0.16kcal∕mol .