## Abstract

Using a large, balanced one-electron basis set, fully optimized reaction space (FORS) calculations to optimize the orbitals and to estimate the internal correlation energy, multi-reference configuration interaction calculations including all single and double excitations out of the FORS reference space to estimate a fraction of the external correlation energy, and the method of scaled external correlation (SEC), we calculate the interaction energy of F with H_{2} in the vicinity of the saddle point for the reaction F + H_{2} → HF + H. Our calculated barrier height, 1.6 kcal/mol, is considerably lower than values obtained in recent ab initio calculations, and the saddle point geometry is about 0.3 a_{0} looser. This indicates that the part of the external correlation energy omitted from MR CISD calculations because of the incompleteness of the one-electron basis set and the truncation of the CI expansion, as estimated by the SEC method, has a significant effect on both the saddle point energy and its geometry.

Original language | English (US) |
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Pages (from-to) | 475-478 |

Number of pages | 4 |

Journal | Chemical Physics Letters |

Volume | 121 |

Issue number | 6 |

DOIs | |

State | Published - Nov 22 1985 |

### Bibliographical note

Funding Information:The authors are grateful to E-R_ Davidson and T H Dunmng Jr for helpful discussion This work was supported in part by the National Science Foun-

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