An efficient formulation and implementation of the analytic energy gradient method to the single and double excitation coupled-cluster wave function: Application to Cl2O2

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Abstract
The analytic energy gradient for the single and double excitation coupled-cluster (CCSD) wave function has been reformulated and implemented in a new set of programs. The reformulated set of gradient equations have a smaller computational cost than any previously published. The iterative solution of the linear equations and the construction of the effective density matrices are fully vectorized, being based on matrix multiplications. The new method has been used to investigate the Cl2O2 molecule, which has recently been postulated as an important intermediate in the destruction of ozone in the stratosphere. In addition to reporting computational timings, the CCSD equilibrium geometries, harmonic vibrational frequencies, infrared intensities, and relative energetics of three isomers of Cl2O2 are presented. The relative energies of the three isomers are further investigated using large atomic natural orbital basis sets in conjunction with the CCSD(T) method, which includes a perturbational estimate of connected triple excitations. The peroxide form of Cl2O2 is predicted to be the lowest energy isomer with the ClClO2 form lying 5.1±3.5 kcal/mol higher in energy.