## Abstract

In calculations on open-shell systems, spin-flipped geminals must be used in explicitly correlated F12 methods with spin-unrestricted or restricted open-shell Hartree-Fock reference wave functions. We suggest the use of the sp ansatz of Bokhan et al. [J. Chem. Phys. 131, 084105 (2009)] for calculations with fixed F12 amplitudes, but to use a novel contracted geminal approach if the F12 amplitudes are to be optimised. This new approach is denoted MP2-F12-o (CCSD(F12)-o). The performance of the new approach is assessed by calculating the atomisation energies of a test set comprising 106 molecules containing the atoms H, C, N, O and F, and by calculating the ionisation energies and electron affinities of the atoms C, N, O and F.

Original language | English |
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Pages (from-to) | 315-325 |

Number of pages | 11 |

Journal | Molecular Physics |

Volume | 108 |

Issue number | 3-4 |

DOIs | |

Publication status | Published - 2010 |

## Keywords

- open-shell systems
- electron correlation
- geminals
- correlation cusp
- AUXILIARY BASIS-SETS
- ZETA BASIS-SETS
- 2ND-ORDER PERTURBATION-THEORY
- CLUSTER CORRELATION ENERGIES
- ELECTRONIC-STRUCTURE THEORY
- CORRELATION CUSP
- FORMULATION