Abstract
It is not always possible in Kohn–Sham density-functional theory for the non-interacting reference state to have integer-only occupancies. Cases of “strong” correlation, with very small HOMO-LUMO gaps, involve fractional occupancies. At the transition states of symmetric avoided-crossing reactions, for example, representation of the correct density requires a 50/50 mixing of degenerate HOMOs. In a recent paper (Becke, J Chem Phys 139:021104, 2013) the “B13” strong-correlation density functional of Becke (J Chem Phys 138:074109, 2013 and 138:161101, 2013) was shown to give excellent barrier heights in symmetric avoided-crossing reactions. However, the calculations were performed only at reactant and transition-state geometries, where the fractional HOMO-LUMO occupancies in the latter are 50/50 by symmetry. In the present chapter, we compute full reaction curves for avoided crossings in H2 + H2, ethylene (twisting around the double bond), and cyclobutadiene (double-bond automerization) by determining fractional occupancies variationally. We adopt a practical strategy for doing so which does not involve self-consistent B13 computations (not yet possible) and involves minimal cost. Single-bond dissociation curves for H2 and LiH are also presented.
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Becke, A.D. (2014). Fractional Kohn–Sham Occupancies from a Strong-Correlation Density Functional. In: Johnson, E. (eds) Density Functionals. Topics in Current Chemistry, vol 365. Springer, Cham. https://doi.org/10.1007/128_2014_581
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DOI: https://doi.org/10.1007/128_2014_581
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