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## Abstract

We examine the magnitude and the basis set convergence of post-coupled-cluster with single, double, and perturbative triple excitations (CCSD(T)) contributions (up to CCSDTQ567) for a wide and diverse set of 21 first- and second-row molecules with up to four non-hydrogen atoms. We focus on multireference systems for which post-CCSD(T) effects are particularly pronounced. The considered molecules are BN(^{1}∑^{+}), C_{2}(^{1}∑^{+}), O_{2}, FO, P_{2}, S_{2}, ClO, N_{2}O, NO_{2}, O_{3}, FNO, FO_{2}, F_{2}O, S_{2}O, S_{3}, ClNO, ClOO, Cl_{2}O, N_{2}C_{2}, P_{4}, and S_{4}. This set spans the gamut from molecules dominated by moderate nondynamical correlation (e.g., FO, ClO, NO_{2}, S_{2}O, N_{2}C_{2}, and P_{4}) to systems dominated by strong nondynamical correlation (e.g., BN, C_{2}, FO_{2}, O_{3}, ClOO, and S_{4}). We examine the basis set convergence of the CCSDT, CCSDT(Q), CCSDTQ, CCSDTQ(5), CCSDTQ5, CCSDTQ5(6), CCSDTQ56, CCSDTQ56(7), and CCSDTQ567 methods. The largest basis sets employed in each category are cc-pV6Z (CCSDT(Q)), cc-pV5Z (CCSDTQ), cc-pVTZ (CCSDTQ5(6)), and cc-pVDZ (CCSDTQ567). Apart from examining the basis-set convergence of post-CCSD(T) contributions near the one-particle basis-set limit, this work explores cost-effective approaches for obtaining these contributions from fairly small basis sets. We consider both effective basis-set extrapolations and scaling factors. An important finding is that extrapolating the perturbative connected quadruples, (Q), from the cc-pVDZ(4s3p1d) and cc-pVTZ basis sets yields near basis-set limit results and represents a significant improvement relative to cc-pV{D,T}Z extrapolation at no additional computational cost (where cc-pVDZ(4s3p1d) is an extended version of the cc-pVDZ basis set). Combining the (Q)/cc-pV{D(4s3p1d),T}Z extrapolations with the fully iterative connected quadruples, Q-(Q), contribution calculated with the cc-pVDZ (or even the cc-pVDZ(3s2p)) basis set is a cost-effective way for obtaining the connected quadruples component close to the basis-set limit (where cc-pVDZ(3s2p) is a truncated version of the cc-pVDZ basis set). In addition, we show that the (5)/cc-pVDZ(3s2p) and (6)/cc-pVDZ(3s2p) components provide reasonable approximations for the connected quintuple and sextuple components close to the basis-set limit, respectively.

Original language | English |
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Article number | 034102 |

Journal | Journal of Chemical Physics |

Volume | 149 |

Issue number | 3 |

DOIs | |

Publication status | Published - 21 Jul 2018 |

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