Diabatic valence-hole states in the C${}_2$ molecule: "Putting Humpty Dumpty together again"

Abstract

Despite the long history of spectroscopic studies of the C${}_2$ molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C${}_2$ is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here to disentangle the numerous configuration interactions within four symmetry manifolds of C${}_2$ (${}^1{\Pi }_g$, ${}^3{\Pi }_g$, ${}^1{\Sigma }_u^{+}$, and ${}^3{\Sigma }_u^{+}$). The key concept of our model is the existence of two "valence-hole" configurations, $2{\sigma }_g^{2}2{\sigma }_u^{1}1{\pi }_u^{3}3{\sigma }_g^2$ for ${}^{1,3}{\Pi }_g$ states and $2{\sigma }_g^{2}2{\sigma }_u^{1}1{\pi }_u^{4}3{\sigma }_g^1$ for ${}^{1,3}{\Sigma }_u^{+}$ states that derive from $3{\sigma }_g\leftarrow 2{\sigma }_u$ electron promotion. The lowest-energy state from each of the four C${}_2$ symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. As a result of their large binding energy (nominal bond order of 3) and correlation with the 2s${}^2$2p${}^2$+2s2p${}^3$ separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the $global$ electronic structure and unimolecular dynamics of C${}_2$.

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• Orbital Optimization in the Density Matrix Renormalization Group, with applications to polyenes and β-carotene
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Cited by in corpus (1)

• Multireference configuration interaction study of the predissociation of C${}_2$ via its $F{}^1{\Pi }_u$ state