chemissian.com

Chemissian is a desktop tool for analyzing the electronic structure and spectra of molecules, and it produces publication-ready figures in five main areas: molecular orbital (MO) energy-level diagrams, MO contribution diagrams, UV-Visible spectra overlaid with experimental data, electronic and spin density maps, and population/valence results. It works by reading output files from quantum chemistry packages such as Gaussian, GAMESS/US-Gamess, Firefly/PC-Gamess, Q-Chem, Molpro, NWChem, ORCA, Turbomole, and Spartan, so the analyses you can run depend on what data those outputs contain (for example, TDDFT or CIS results are needed for spectra).

Molecular orbital energy-level diagrams

Chemissian builds MO diagrams from both Hartree-Fock and Kohn-Sham calculations. The diagram is not just a static plot: an integrated graphical editor lets you add text labels, draw connecting lines between energy levels, and "occupy" levels with electrons. You can move between energy levels with the keyboard or mouse cursor to inspect each one, which makes the diagrams suitable for explaining bonding and electronic structure in a paper or presentation.

Contribution diagrams

For any selected MO, Chemissian shows how much each atomic or molecular fragment contributes. This appears either as a separate contribution diagram or directly on the MO diagram. This is the feature to use when you need to argue that a particular orbital is localized on one fragment or delocalized across the molecule.

UV-Visible spectra

Chemissian includes a graphical analyzer for electronic spectra. Load an output file containing TDDFT or CIS data from GAMESS, Firefly, Gaussian, Spartan, Q-Chem, NWChem, or ORCA, and the spectrum is displayed. Calculated and experimental UV-VIS spectra can be plotted on the same chart, which supports assigning observed bands to computed transitions and studying the nature of those transitions.

Electronic and spin density maps

The software calculates and visualizes electronic density and spin density, and prepares the images for publication. It also handles spin natural orbitals and natural transition orbitals, which are useful for describing excited states and open-shell systems in a compact orbital picture.

Populations and valences

Chemissian calculates populations and valences, giving quantitative backing to the orbital pictures above — for example, when you want to compare charge distribution or bonding character across a series of molecules.

Practical notes before you start

• Check your output type first. Spectra require TDDFT/CIS data in the output; MO diagrams require the orbital energies and coefficients that the supported packages write by default.
• Confirm package support. The site lists Gaussian, US-Gamess, Firefly/PC-Gamess, Q-Chem, Molpro, NWChem, ORCA, Turbomole, and Spartan, but the spectra section names a slightly narrower set (GAMESS, Firefly, Gaussian, Spartan, Q-Chem, NWChem, ORCA). If you use Molpro or Turbomole, verify that the specific analysis you need is available.
• Plan for figure editing. Labels, connecting lines, and electron occupancy are added inside Chemissian, so you can finish diagrams there rather than exporting to a separate drawing tool.

The site provides a download page, an order page, and online help; pricing and licensing terms are not stated in the material reviewed here, so check the Order page directly if cost matters for your decision.

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

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.

Chemissian can read output files from Gaussian, US-GAMESS, Firefly/PC-GAMESS, Q-Chem, Molpro, NWChem, ORCA, Turbomole, and Spartan. For UV-VIS spectrum work, the file must contain TDDFT or CIS results; for molecular orbital (MO) energy-level diagrams, the output needs Hartree-Fock or Kohn-Sham data. The program's graphical interface is built to examine and visualize data from these files, so in practice you load the output produced by one of these packages and work with it inside Chemissian rather than converting it first.

Supported Programs and What Each Is Used For

The table below groups the packages by the tasks the site describes. All of them are listed as sources Chemissian can read.

The site's feature descriptions name GAMESS, Firefly (PC-GAMESS), GAUSSIAN, Spartan, NWChem, Orca, and Q-Chem specifically for spectrum building. Molpro and Turbomole appear in the general list of supported output formats.

What the Output File Must Contain

Support depends on the calculation type, not just the program name:

• MO energy-level diagrams require Hartree-Fock or Kohn-Sham results, since Chemissian builds diagrams with both methods.
• UV-VIS spectra require TDDFT or CIS data in the output. A file from a supported program without these results will not produce a spectrum.
• Comparison with experiment works by plotting calculated and experimental UV-VIS spectra on the same graph, so you need both the calculated output and your experimental spectrum data.

If a file loads but a feature is unavailable, check the calculation type before assuming the format is unsupported.

What You Can Do Once the File Is Loaded

• MO diagrams: add text labels, connect energy levels with lines, "occupy" levels with electrons, and navigate between levels by keyboard or mouse. Contributions from atomic or molecular fragments can be shown on a contribution diagram or directly on the MO diagram.
• UV-VIS analysis: build and visualize spectra, study the nature of transitions, and place calculated and experimental spectra on one plot.
• Density and orbitals: calculate and visualize spin natural orbitals, natural transition orbitals, and electronic and spin densities, prepared for publication.
• Populations and valences: calculate populations and valences from the imported data.

Practical Notes Before You Start

• Confirm your program is on the list above. Outputs from packages not named on the site are not described as supported.
• For spectra, verify the job actually wrote TDDFT or CIS results; this is the most common reason a spectrum cannot be built.
• The site offers a Download page, an Order page, and Online Help, so you can check availability and licensing there before committing to a workflow. Pricing and access terms are not stated in the material reviewed here, so confirm them on those pages rather than assuming free use.

If your goal is simply to know whether Chemissian fits your existing workflow, the deciding question is whether your calculations come from one of the nine listed packages and include the relevant results (HF/Kohn-Sham for MO diagrams, TDDFT/CIS for spectra).

Chemissian is a desktop tool for interpreting molecular electronic structure and spectra. It helps with UV-VIS analysis by building spectra from TDDFT/CIS output files, plotting calculated and experimental spectra on the same graph, and examining the nature of electronic transitions. For bonding, it builds molecular orbital (MO) energy-level diagrams from Hartree-Fock or Kohn-Sham results, shows fragment contributions to each orbital, and calculates populations and valences. It is most useful if your calculations come from a supported quantum-chemistry package and you want publication-ready figures without writing your own plotting scripts.

Building and Comparing UV-VIS Spectra

Chemissian reads output files that already contain TDDFT or CIS data and turns them into spectra you can inspect and export.

• Input: an output file from GAMESS, Firefly (PC-GAMESS), Gaussian, Spartan, Q-Chem, NWChem, or ORCA that includes TDDFT/CIS excited-state data.
• Action: load the file into the graphical analyzer and build the UV/VIS spectrum.
• Expected result: a plotted spectrum you can visualize and interpret.

The practical advantage is side-by-side comparison: calculated and experimental spectra can be placed on the same plot. That lets you check whether computed transition energies and intensities line up with measured peaks, and then dig into which transitions produce each feature.

Studying the Nature of Transitions

Beyond plotting, Chemissian is built to examine what a transition actually consists of. You can analyze MO composition and use that to attribute spectral features to specific orbitals and fragments. The site also lists support for spin natural orbitals and natural transition orbitals, which are commonly used to describe excited states more compactly than raw MO transitions.

Analyzing Bonding Through Orbital Diagrams

For bonding questions, the core output is the MO energy-level diagram.

• Diagrams can be built with either Hartree-Fock or Kohn-Sham methods.
• A graphical editor lets you add text labels, draw connecting lines between energy levels, and "occupy" levels with electrons.
• You can navigate between energy levels with the keyboard or mouse.
• Contributions to the current MO from atomic or molecular fragments are shown on a contribution diagram or directly on the MO diagram.

That last point is what makes the tool useful for bonding arguments: instead of only seeing orbital energies, you see which fragments contribute to an orbital, which supports statements about bonding character. Chemissian also calculates populations and valences and can calculate and visualize electronic and spin densities, preparing them for publication.

Supported Programs and Practical Fit

Chemissian's graphical interface is designed to examine and visualize output from Gaussian, US-GAMESS, Firefly/PC-GAMESS, Q-Chem, Molpro, NWChem, ORCA, Turbomole, and Spartan. Note that the spectrum-building description specifically names GAMESS, Firefly, Gaussian, Spartan, Q-Chem, NWChem, and ORCA for TDDFT/CIS data, so confirm your file type matches the analysis you need before relying on it.

Choose Chemissian if you already run these packages and want one interface for MO diagrams, density maps, and calculated-versus-experimental spectra. If your workflow is entirely script-based or your output format is not listed, the tool's value depends on whether your files can be read at all. The site provides a Download page, an Order page, and Online Help, so check format compatibility and licensing terms there before committing.

Chemissian is available from its official site, chemissian.com. The site's navigation includes Download, Order, Online Help, FAQ, News and updates, and Contact us pages, so you can get the software and its documentation from the same place. Use the Download page to obtain the program, the Online Help and FAQ pages for usage questions, and the Order page for licensing.

Getting the Software

The site lists a dedicated Download page in its main navigation. That is the entry point for obtaining Chemissian. The site also has an Order page, which indicates that licensing or purchase is handled separately from the download; the available materials do not state prices or whether the download itself is unrestricted, so check both pages before assuming terms.

Documentation and Support

The Online Help page is the main documentation resource, and the FAQ page covers recurring questions. The News and updates page carries current information about the software, which is worth checking when you want to know whether a feature or fix you need has been released.

What the Documentation Covers

Knowing what Chemissian does helps you target the right help page. According to the site, Chemissian analyzes the electronic structure and spectra of molecules. Its documented capabilities include:

• Building molecular orbital energy-level diagrams using Hartree-Fock and Kohn-Sham methods, with a graphical editor for text labels, connecting lines between levels, and placing electrons on levels.
• Comparing calculated and experimental UV-VIS electronic spectra on the same plot.
• Building, visualizing, and interpreting UV-VIS spectra from TDDFT/CIS output.
• Calculating and visualizing spin natural orbitals, natural transition orbitals, and electronic and spin densities, prepared for publication.
• Analyzing electronic density distribution and calculating populations and valences.

If your question concerns one of these tasks, the Online Help page is the place to look first; the FAQ is better suited to setup or recurring-issue questions.

Input Files You Can Bring

A practical check before downloading: Chemissian reads output files from a range of quantum chemistry packages, including Gaussian, US-Gamess, Firefly/PC-Gamess, Q-Chem, Molpro, NWChem, ORCA, Turbomole, and Spartan. The UV-VIS spectrum tools are described for output from GAMESS, Firefly (PC-GAMESS), GAUSSIAN, Spartan, NWChem, Orca, and Q-Chem containing TDDFT/CIS data. Confirm your program is on these lists so the file you plan to load is supported.

A Reasonable First Session

1. Open chemissian.com and go to the Download page to get the software.
2. Install it, then open Online Help and read the sections matching your task, such as MO diagrams or UV-VIS spectra.
3. Load an output file from a supported package that contains the relevant data (for spectra, TDDFT/CIS results).
4. If something behaves unexpectedly, check the FAQ before using Contact us.
5. Check News and updates periodically for changes that affect your workflow.

The main snag to expect is loading a file without the data a feature needs, for example a spectrum tool applied to output with no TDDFT/CIS results. Verifying the calculation type in your output before loading avoids most of these dead ends.