A class IV charge model for molecular excited states

We present a new parameterization for calculating class IV charges for molecules containing H, C, N, O, F, Si, P, S, Cl, Br, and I from wave functions calculated at the intermediate-neglect-ofdifferential-overlap-for-spectroscopy (INDO/S) level. First we readjust the oxygen parameters in INDO/S on the basis of electronic excitation energies; this yields a new set of parameters called INDO/S2. Then we parameterize the charge model. The new model, called charge model 2 for INDO/S2 (CM2/INDO/S2), is parameterized against the most accurate available data from both ab initio and experimental sources for dipole moments of ground and excited electronic states. For a training set containing 211 dipole moments of molecules in their ground states and 33 dipole moments of molecules in their first excited states, the CM2/INDO/S2 model leads to a root-mean-square (rms) error in dipole moments of 0.26 D for ground states and 0.40 D for the excited states. The new model, INDO/S2 with CM2, systematically improves the n→π* excitation energies and the dipole moments of the excited states of carbonyl compounds. We also parameterized a CM2 model for the standard INDO/S model (CM2/INDO/S), which predicts quite accurate dipole moments for ground states with an rms error of 0.24 D.