Electron Correlation on Calculated Infrared Intensities

Influence of Electron Correlation on Calculated Infrared Intensities 

Consistent efforts have been focused over the past decade on the development of meUiods for incorporation of electron correlation in evaluating molecular properties. Analytical derivative approaches have been formulated for multiconfiguration SCF (MC SCF) wave functions [168,214-218], configuration interaction (Cl) wave functions [219- 

coupled-cluster (CC) [222-224] and many body perturbation theories (MBPT) [181,225,226]. 

The results presented in Table 7.5 show that higher level of SGF and electron correlation calculations improve in a quite systematic way the accord between predicted and observed infrared intensities. A nice agreement vnth experiment is achieved from DZP+dSPD/DMG 6/6 calculations (last column of Table 7.5). Gonsidering just one example does not, permit any definite conclusions. Moreover, as pointed out by Amos [175], MG SGF calculations account for only a fraction of the electron correlation. 

The many body perturbation theory has been applied to obtain higher accuracy in ab initio calculations of molecular properties. Pople etal. [181] have developed analytic derivative methods at second order perturbation theory level (MBPT(2)). Simandiras et al. [184] have derived specific expressions for analytic determination of dipole moment derivatives at MBPT(2). Dierksen and Sadlej [229] have shown by applying finite field MBPT in studying dipole and quadruple polarizabilities of the CO molecule that fourth and even higher level of MBPT is required to achieve satisfactory results. 

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B. Influence of Electron Correlation on Calculated Infrared Intensities... 

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