Analytic energy gradients for open-shell coupled-cluster singles and doubles (CCSD) calculations using restricted open-shell Hartree—Fock (ROHF) reference functions

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The theory of analytic energy gradients for the open-shell single and double excitation coupled-cluster (CCSD) method based on restricted open-shell Hartree—Fock (ROHF) reference functions is presented. The new CCSD gradient method is applied to the dissociation of the 3A″ state of formaldehyde (CH 2O) to H and HCO. Complete geometry optimization is carried out for the initial reactant, the transition state of the reaction, and for the dissociation products. Non-iterative triples appropriate to ROHF are introduced and are used to define a new ROHF-CCSD(T) method. Using this approach and zero-point corrections, the activation energy is calculated to be 21.0 kcal/mol and the exit barrier height is predicted to be 6.1 kcal/mol, both in excellent agreement with experiment.