GOUTERMAN - A module for simulations of tetrapyrrole optical spectra.

Tetrapyrrole macrocycles comprise the pigments of life (heme, chlorophylls, etc.) and together with synthetic analogues also find wide use in materials chemistry and the life sciences. The distinct colors of the various macrocycles are essential features. A time-honored approach for understanding the electronic spectra entails analyzing the impact of molecular composition on the relative energies of the four frontier molecular orbitals (MOs) of these 18-π annulenes. While first introduced by Prof. Martin Gouterman in 1959, use of the four-orbital model has been largely conceptual. Here, a computational module (GOUTERMAN) is introduced that takes as input the energies of the four MOs, provides user control over key parameters (configuration-interaction energies, dipole strengths, spectral widths), and outputs simulated absorption spectra. The energies of the MOs can be calculated independently (e.g., from density functional theory) or drawn from a small database that is provided. The MO energies and key parameters can be toggled and spectral changes viewed. The GOUTERMAN module is available in PhotochemCAD as a tool for both specialists and non-specialists to visualize, via hands-on experience, the connections between the structural and spectral properties of the tetrapyrrole chromophores found ubiquitously in Nature and used widely in research.