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Updated: Aug 20, 2026

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
Published on: April 8, 2020
Convergence of triples energy in CCSD(T) and CC3 calculations with correlation-consistent basis sets
Eduardo Fischli Laschuk1, Paolo Roberto Livotto
1Instituto de Química, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil. fischli@iq.ufrgs.br
Abstract:
The convergence behavior of connected triples correlation energy in CCSD(T) and CC3 calculations with (aug-)cc-pVXZ basis sets has been accurately described in terms of a power law of the type E(X)=E(infinity)+AX(-4). Calculations ranging from double-Z through septuple-Z attest the validity of this X(-4) convergence model. Extrapolations generated from (X-1,X)-Z calculations yield energies of nearly (X+1)-Z quality. Typically, the fraction of triples correlation energy recovered is 0.92+/-0.05 in (D,T) extrapolations; 0.98+/-0.01 in (T,Q) extrapolations; 1.0002+/-0.0012 in (Q,5) extrapolations; and 0.9995+/-0.0005 in (5,6) extrapolations.
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