ANALYTIC ENERGY GRADIENTS FOR OPEN-SHELL COUPLED-CLUSTER SINGLES AND DOUBLES (CCSD) CALCULATIONS USING RESTRICTED OPEN-SHELL HARTREE-FOCK (ROHF) REFERENCE FUNCTIONS

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 (CH2O) 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.