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

Abstract

The notoriously small $X^3\Pi -a^1{\Sigma }^{+}$ excitation energy of the BN diatomic has been calculated using high-order coupled cluster methods. Convergence has been established in both the 1-particle basis set and the coupled cluster expansion. Explicit inclusion of connected quadruple excitations ${\hat{T}}_4$ is required for even semiquantitative agreement with the limit value, while connected quintuple excitations ${\hat{T}}_5$ 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 $T_e$ by about 60 cm${}^{-1}$ at the CCSDT, and 85 cm${}^{-1}$ at the CCSDTQ level. Our best estimate, $T_e$=183$\pm$40 cm${}^{-1}$, is in excellent agreement with earlier calculations and experiment, albeit with a smaller (and conservative) uncertainty. The dissociation energy of BN($X^3\Pi$) is $D_e$=105.74$\pm$0.16 kcal/mol and $D_0$=103.57$\pm$0.16 kcal/mol.

J. Chem. Phys., in press

References in corpus (1)

• W3 theory: robust computational thermochemistry in the kJ/mol accuracy range

Cited by in corpus (1)

• Basis set convergence of post-CCSD contributions to molecular atomization energies