NDDO‐Based CI Methods for the Prediction of Electronic Spectra and Sum‐Over‐States Molecular Hyperpolarization

NDDO‐based (AM1) configuration interaction (CI) calculations have been used to calculate the wavelength and oscillator strengths of electronic absorptions in organic molecules and the results used in a sum‐over‐states treatment to calculate second‐order hyperpolarizabilities. The results for both spectra and hyperpolarizabilities are of acceptable quality as long as a suitable CI‐expansion is used. We have found that using an active space of eight electrons in eight orbitals and including all single and pair‐double excitations in the CI leads to results that agree well with experiment and that do not change significantly with increasing active space for most organic molecules. Calculated second‐order hyperpolarizabilities using this type of CI within a sum‐over‐states calculation appear to be of useful accuracy.