Oscillator strengths for D4h and Dzh porphyrins are calculated using the dipole gradient formulation and compared with calculations employing the dipole length operator. The effect of conEiguration interiction is also discussed.
Interest has recently developed in the use of the gradient operator V as an alternative to the position operator r in calculations of electric dipole allowed intensities in molecular spectra. Early work of Berthier [l] and Pullman [2] was followed by a series of papers by Chong ISI, Chong and Yue [4] and McHugh and Gouterman [5], who showed that for the lower-energy singlet-singlet excitations of polyenes, (Y -w diphenyl polyenes and polyacenes, intensity calculations using the gradient operator in general give significantly better agreement with experiment. These calcul.at.ions have been made using s-electron eigenfunctions, and the improved agreement with experiment of theoretical intensities is fouhd for calculations based on simple HfIckel functions ar!d also SCF ZDO functions. De Bruijn [6] has recently shown that the lowest triplet-triplet intensities of a series of polyacenes can also be calclllated with reasonable accuracy using the gradient operator *.
Following Chong et al. [3,4], we refer to oscillator strengths calculated by means of position and