SEMIEMPIRICAL VALENCE-ELECTRON CALCULATIONS OF EXCITED-STATE GEOMETRIES AND VIBRATIONAL FREQUENCIES

The use of finite differences and finite second differences in order to approximate gradients and second derivatives of the energy for geometry optimization and determination of normal modes of vibration on the CI level of computation is discussed in connection with the semiempirical MNDOC-CI valence electron method. Results are given for ground and excited states of ethylene, acetylene, formaldehyde, acetaldehyde, acetone, formamide and acetamide and are compared with experimental and ab initio data. Mean absolute errors for bond lengths, bond angles, excitation energies and vibrational frequencies indicate that the MNDOC-CI method is well suited to describe ground and excited states of organic molecules on the same level of approximation and with comparable accuracy.