Determination of ab initio absolute Raman excitation profiles using linear response theory

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AbstractA study of the computational requirements for the calculation of Raman‐related properties is presented, and the importance of the inclusion of electronic correlation and basis sets on the convergence of absolute Raman intensities and depolarization ratios for a set of small polyatomic systems (H2O, NH3, CH4, and C2H2) is discussed. These properties are derived from the geometric gradients of the dynamic polarizabilities evaluated with linear response equations. The Complete Active Space Self‐Consistent Field (CASSCF) static correlation effects as well as the effects of dynamic electronic correlation at the Møller‐Plesset Second‐Order Perturbation Theory (MP2), approximate Coupled Cluster Singles and Doubles (CC2), and Coupled Cluster Singles and Doubles (CCSD) levels on these properties were investigated and are presented. Basis set convergence for the series aug‐cc‐pV(D,T,Q)Z were analyzed. The results obtained using Sadlej's polarizability basis set are also presented. While the MP2 level produces good relative Raman intensities, it is shown that accurate absolute intensities require high‐level correlated wave functions (CCSD). The importance of static correlation effects for the Raman intensities is assessed. Large basis sets are required to predict Raman intensities accurately (aug‐cc‐pVTZ), but Sadlej's basis set is shown to be the best choice to compute these properties for large molecules. Depolarization ratios showed little dependency on the basis set and the theoretical level. It was determined that the correlation effects on geometric gradients of polarizabilities are associated mainly with molecular polarizabilities, and not with normal coordinates. The optimal size of the atomic displacements used to compute numerical derivatives of polarizabilities without influencing the Raman properties derived from them was ascertained. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005