title: Importance of Triples Contributions to NMR Spin-Spin Coupling Constants Computed at the CC3 and CCSDT Levels.
abstract: We present the first analytical implementation of CC3 second derivatives using the spin-unrestricted approach. This allows, for the first time, the calculation of nuclear spin-spin coupling constants (SSCC) relevant to NMR spectroscopy at the CC3 level of theory in a fully analytical manner. CC3 results for the SSCCs of a number of small molecules and their fluorine substituted derivatives are compared with the corresponding coupled cluster singles and doubles (CCSD) results obtained using specialized basis sets. For one-bond couplings the change when going from CCSD to CC3 is typically 1-3%, but much higher corrections were found for1JCNin FCN, 15.7%, and1JOFin OF2, 6.4%. The changes vary significantly in the case of multibond couplings, with differences of up to 10%, and even 13.6% for3JFHin fluoroacetylene. Calculations at the coupled cluster singles, doubles, and triples (CCSDT) level indicate that the most important contributions arising from connected triple excitations in the coupled cluster expansion are accounted for at the CC3 level. Thus, we believe that the CC3 method will become the standard approach for the calculation of reference values of nuclear spin-spin coupling constants.
authors.affiliation: Department of Chemistry, University of Copenhagen , Universitetsparken 5, 2100 Copenhagen Ø, Denmark.
authors.forename: Rasmus
authors.identifier: http://orcid.org/0000-0001-8524-0684
authors.initials: R
authors.lastname: Faber
authors.affiliation: Department of Chemistry, University of Copenhagen , Universitetsparken 5, 2100 Copenhagen Ø, Denmark.
authors.forename: Stephan P A
authors.identifier: http://orcid.org/0000-0003-4812-0522
authors.initials: SP
authors.lastname: Sauer
authors.affiliation: Institut für Physikalische Chemie, Universität Mainz , 55099 Mainz, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Gauss