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# ATOMIC‑2 Protocol for Thermochemistry

dataset

posted on 2022-06-03, 21:29 authored by Dirk BakowiesATOMIC
is a midlevel thermochemistry protocol that uses Pople’s
concept of bond separation reactions (BSRs) as a theoretical framework
to reduce computational demands in the evaluation of atomization energies
and enthalpies of formation. Various composite models are available
that approximate bond separation energies at the complete-basis-set
limit of all-electron CCSD(T), each balancing computational cost with
achievable accuracy. Evaluated energies are then combined with very
high-level, precomputed atomization energies of all auxiliary molecules
appearing in the BSR to obtain the atomization energy of the molecule
under study. ATOMIC-2 is a new version of the protocol that retains
the overall concept and all previously defined composite models but
improves on ATOMIC-1 in various other ways: Geometry optimization
and zero-point-energy evaluation are performed at the density functional
level (PBE0-D3/6-311G(d)), which shows significant computational savings
and better accuracy than the previously employed RI-MP2/cc-pVTZ. The
BSR framework is improved, using more accurate complete-basis-set
(CBS) extrapolations toward the Full CI limit for the atomization
energies of all auxiliary molecules. Finally, and most importantly,
an error and uncertainty model termed ATOMIC-2

_{um}is added that estimates average bias and uncertainty for each of the atomization energy contributions that arise from the simplified treatment of some contributions to bond separation energies (CCSD(T)) and the neglect of others (such as higher order, scalar relativistic, or diagonal Born–Oppenheimer corrections) or from residual error in the energies of auxiliary molecules. Large and diverse benchmarks including up to 1179 molecules are used to evaluate necessary reference data and to correlate the observed error for each of the contributions with appropriate proxies that are available without additional quantum-chemical calculations for a particular molecule and represent its size and type. The implementation of ATOMIC-2 considers neutral, closed-shell molecules containing H, C, N, O, and F atoms; compared to ATOMIC-1, the framework has been extended to cover a few challenging but rare bond topologies. In comparison to highly accurate reference data for 184 molecules taken from the ATcT database (V. 1.122r), regular ATOMIC-2 shows noticeable underbinding, but the bias-corrected protocol ATOMIC-2_{um}is found to be more accurate than either ATOMIC-1 or standard Gaussian-4 theory, and the uncertainty model is consistent with statistics of actually observed errors. Problems arising from ambiguous or challenging Lewis-valence structures defining BSRs are discussed, and computational efficiency is demonstrated. Computer code is made available to perform ATOMIC-2_{um}analyses.## History

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uses pople ’rare bond topologiespreviously employed ridiverse benchmarks includingbond separation reactionsreduce computational demandsbalancing computational costfull ci limitactually observed errors184 molecules takenbond separation energiesvarious composite modelsum estimates average biasdensity functional levelauxiliary molecules appearing2 considers neutralmidlevel thermochemistry protocolelectron ccsd (precomputed atomization energiescorrected protocol atomicatomization energy contributionsauxiliary moleculesccsd (atomization energyatomization energiescomputational efficiencythermochemistry atomic1179 molecules311g (evaluated energiesobserved errorstandard gaussiansimplified treatmentset limitscalar relativisticproblems arisingperform atomicnew versionhigher ordergeometry optimizationf atomsextrapolations towardeither atomiccomputer codechemical calculationsbetter accuracyatct databaseappropriate proxiesachievable accuracy4 theory

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