The effect of higher than double excitations on the F+H2→FH+H barrier

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Abstract
The classical barrier height and saddle point geometry are computed using the averaged coupled‐pair functional (ACPF) method, correlating both seven and nine electrons. The size‐consistent ACPF method indicates that 2s correlation substantially reduces the barrier. The excellent agreement between the ACPF and MRCI+Q results at both the seven and nine electron level provides additional support for the +Q correction. Thus the ACPF treatment supports a low (1.65 kcal/mol) barrier. The seven‐electron CISDTQ results are shown to be consistent with the MRCI+Q and ACPF results.