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WIREs Comput Mol Sci
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Homodesmotic reactions for thermochemistry

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Abstract Achieving highly accurate thermochemical predictions from relatively low levels of electronic structure theory has long been a goal of computational chemistry. One route to such a goal is to exploit the systematic cancellation of errors. This approach was spearheaded by the Pople group in the 1970s with the introduction of isodesmic bond‐separation reactions, and then extended by other groups to homodesmotic (HD) and hyperhomodesmotic reactions over the ensuing years. Unfortunately, the propagation of multiple, nonequivalent definitions of HD reactions, accompanied by a panoply of related reaction classes, has lead to a great deal of confusion. We review recent efforts to introduce a refined hierarchy of HD reactions for use in thermochemical predictions and to quantify the degree of error cancellation that can be achieved. Examples of the use of reactions from the HD hierarchy from the literature are presented, as are current limitations of this HD hierarchy for thermochemistry. Although the use of such error‐balanced reactions are no longer mandatory for high‐accuracy thermochemical predictions of small molecules, they still offer significant advantages in this context, and offer one route to accurate thermochemical predictions of larger molecules. © 2011 John Wiley & Sons, Ltd. This article is categorized under: Structure and Mechanism > Molecular Structures

Schematic of alternative routes to the enthalpy of formation of 1,3‐pentadiene: (a) atomization reaction and (b) error‐balanced HD reaction.

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Errors versus experiment (kcal mol−1) in enthalpies of formation for n‐alkanes (CnH2n+2) based on atomization energies and ID and HD BSRs. Data from Ref 86.

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(a) Mean errors (kcal mol−1) in energies using popular electronic structure theory methods and (b) mean contributions to accurate FPA energies (EFPA) for isogyric (IG), isodesmic (ID), homodesmotic (HD), and hyperhomodesmotic (HHD) BSRs for a set of 22 nonconjugated hydrocarbons. Data from Ref 5.

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(a) Mean errors (kcal mol−1) in energies using popular electronic structure theory methods and (b) mean contributions to accurate FPA energies (EFPA) for isogyric (IG), isodesmic (ID), hypohomodesmotic (HoHD), homodesmotic (HD), and hyperhomodesmotic (HHD) BSRs for a set of 14 conjugated hydrocarbons. Data from Ref 5.

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Elemental reactants for each reaction class in the HD hierarchy. Data from Ref 5.

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Elemental products for each reaction class within the HD hierarchy. Data from Ref 5.

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Schematic representation of the FPA approach and components of EFPA.

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