A time-dependent semiempirical approach to determining excited states.

We study a time-dependent semiempirical method to determine excitation energies, TD-PM3. This semiempirical method allows large molecules to be treated. A Linear-response Chebyshev approach yields the TD-PM3 spectrum very efficiently. Spectra and excitation energies were tested by comparing it with the results obtained using TD-DFT (Time Dependent-Density Functional Theory), using both small and large basis sets. They were also compared to PM3-CI, Time Dependent-Hartree Fock using the STO-3G basis set, and to experiment. TD-PM3 results generally match better the large-basis set calculations than the small-basis TD-DFT do; excitation energies are almost always accurate to within about 20% or less, except for a few small molecules. Accuracy improves as the molecules get larger.