A New Release of MOPAC Incorporating the INDO/S Semiempirical Model with CI Excited States

We have incorporated the semiempirical INDO/S Hamiltonian into a new release of MOPAC2016, which has long been at the forefront of semiempirical quantum chemical methods (SEQMs). Our new code enables the calculation of excited states using the INDO/S Hamiltonian combined with a configuration interaction approach using single excitations (CIS), single and double excitations (CISD), or multiple reference determinants (MRCI) where reference determinants are generated using a complete active space (CAS) approach. The capacity to perform excited-state calculations beyond the CIS level makes INDO/CI one of the few low-cost computational methods capable of accurately modeling states with substantial double-excitation character. Solvent corrections to the ground-state and excited-state energies can be computed using the COSMO implicit solvent model, incorporating state-specific corrections to the excited states based on the solvent refractive index. We demonstrate that this code produces physically reasonable electronic structures, absorption spectra, and solvatochromic shifts at low computational costs for systems up to hundreds of atoms, and for both organic molecules and metal clusters.