AB-INITIO CALCULATION AND DISPLAY OF THE ROTATORY STRENGTH TENSOR IN THE RANDOM-PHASE-APPROXIMATION - METHOD AND MODEL STUDIES

The theory of the rotatory strength tensor describing circular dichroi sm (CD) of electronic excitations in oriented molecules is summarized in length and velocity formulations, and we outline an ab initio imple mentation of the calculation of this tensor and its electric dipole - magnetic dipole and electric dipole - electric quadrupole components i n the random phase approximation. Results are presented for the predom inantly pi --> pi and sigma --> pi* type transitions in an ethylene m olecule twisted into chiral D-2 conformations demonstrating the origin -sensitivity of the electric dipole-magnetic dipole and electric dipol e-electric quadrupole contributions to the tensor, and demonstrating o rigin-invariance of the total tensor in the velocity formulation. The electric dipole-electric quadrupole contribution is small, but not ins ignificant for both transitions. We present a pictorial representation of the CD response, and its application to the total tensor and its c omponents for the pi --> pi excitation.