What Is Chemissian and What Is It Used For?

Updated

Chemissian is a desktop tool for analyzing the electronic structure and spectra of molecules. It is most useful if you already run quantum chemistry calculations and want to turn output files into molecular orbital (MO) energy-level diagrams, UV-VIS spectra plots, and density or orbital visualizations for interpretation or publication. It reads output from packages such as Gaussian, US-Gamess, Firefly/PC-Gamess, Q-Chem, Molpro, NWChem, ORCA, Turbomole, and Spartan, so it fits a workflow where the calculation is done elsewhere and Chemissian handles the analysis and graphics.

What Chemissian does

Molecular orbital energy-level diagrams

Chemissian builds MO energy-level diagrams from Hartree-Fock and Kohn-Sham results. A built-in graphical editor lets you add text labels, draw connecting lines between levels, and "occupy" levels with electrons. You can move between energy levels with the keyboard or mouse, and the program shows how atomic or molecular fragments contribute to the selected orbital, either on a contribution diagram or directly on the MO diagram.

UV-VIS spectra

If your output file contains TDDFT or CIS data, Chemissian can build the electronic UV/VIS spectrum and display calculated and experimental spectra on the same plot. This is the main way to study the nature of transitions in a UV-VIS spectrum.

Densities and orbitals

The program calculates and visualizes spin natural orbitals, natural transition orbitals, and electronic and spin densities, and prepares these images for publication. It also calculates populations and valences, which supports discussion of bonding character.

Who it fits

Your situation Chemissian is a reasonable fit if…
You already have output files from Gaussian, ORCA, GAMESS, Q-Chem, NWChem, Spartan, Firefly, Molpro, or Turbomole You want MO diagrams, spectra plots, or density/orbital figures without rebuilding them by hand
You need publication-quality MO and orbital figures You want labels, connecting lines, and electron occupancy added inside the tool
You compare theory with experiment You have experimental UV-VIS data to overlay on calculated spectra
You need to run the quantum chemistry calculation itself No — Chemissian analyzes outputs; it is not the calculation engine

Practical notes before you start

  • Check your output format first. The spectrum workflow depends on the file containing TDDFT or CIS data; an output without excited-state results will not produce a spectrum.
  • Plan the analysis, not just the picture. Decide which orbitals or transitions you need to discuss, then use the contribution display to support that point rather than exporting every level.
  • Expect a learning step for the editor. Adding labels, lines, and electron occupancy is manual work; budget time for it when preparing figures.

The site lists Download, Order, FAQ, and Online Help pages, so check those for availability and current terms before committing to it in a project.