The effect of geometry optimization on the reaction barrier height: SCF and MP2 studies of the dissociation and isomerization reactions of formaldehyde

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The structures of formaldehyde and activated complexes of the dissociation and isomerization reactions were optimized at the SCF and the second-order Møller—Plesset (MP2) levels with 6-31G, 6-311G, 6-31G ++, and 6-31G ++ + P′ basis sets. The barrier heights calculated at the MP2 level for the geometry optimized at the SCF level differ from those optimized at the MP2 level only negligibly, while the geometries change significantly. The barrier heights are much more affected by quality of a basis set and inclusion of electron correlation. The high level geometry optimization is necessary only for calculations, where accuracy higher than 6 kJ mol −1 is required.