The effect of triple excitations in coupled cluster calculations of Raman scattering cross-sections

The frequency-dependent linear response approach for the hierarchy of coupled cluster methods CCS/CC2/CCSD/CC3 has been used to evaluate geometric derivatives of polarizabilities and Q-branch Raman scattering cross-sections of five diatomics: N2, CO, HF, HCl and Cl2. The triple excitation corrections are substantial for N2 and CO, where they exceed 10%, and negligible for HF and HCl. The results at the CCSD and CC3 levels with correlation-consistent basis sets have been used to obtain best estimates for the Raman scattering cross-sections. The agreement between the estimated values and experiment is satisfactory, considering the approximations inherent in the harmonic approach and Placzekʹs polarizability theory.