Accuracy and limitations
What a calculated LUMO energy can and cannot tell you, which molecules the tool handles well, and the factors that move the number. Read this before comparing results with literature values.
Orbital energies come from a model, not an experiment.
Same method and basis set, or the comparison is unreliable.
Differences within a series are more robust than single values.
Calculated vs. estimated energy values
| Aspect | Calculated value | Estimated value |
|---|---|---|
| Source | Electronic structure calculation for this molecule | Trend, correlation or experimental proxy |
| Reproducible | Yes, with the same engine and settings | Depends on the source |
| On this site | Every calculator result | Not shown by the calculator |
Experimental quantities such as electron affinities or reduction potentials are related to LUMO energies but are not the same thing. Agreement within a series is common. Exact agreement in absolute value is not expected, and with Extended Hückel theory the absolute values are far from experimental electron affinities.
Supported molecules and input formats
- Neutral organic molecules
- Closed-shell species
- Up to 150 atoms, counting hydrogens
- Conjugated and aromatic systems
- Ions and zwitterions
- Very flexible molecules
- Hypervalent S and P compounds
- Boron compounds (no force field)
- Mixtures and salts as one input
- Radicals and open-shell states
- Polymers and periodic solids
- Metals and elements other than H, B, C, N, O, F, Si, P, S, Cl, Br, I
Accepted inputs are SMILES strings and MOL, SDF and XYZ files. See the Methodology for how input is prepared.
Computational method limitations
Extended Hückel theory (the method used here)
Extended Hückel theory is fast, robust and good at orbital shapes, symmetries and qualitative trends. It is parameterized and not self-consistent, which has consequences for the numbers it produces:
- LUMO energies are typically between −8 and −12 eV for conjugated organic molecules, several electronvolts lower than Hartree-Fock or DFT values for the same molecule.
- Saturated molecules can have positive LUMO energies, which reflects the model rather than a real inability to accept an electron.
- Charge redistribution is not captured, so ions and strongly polar molecules are described less reliably.
- The minimal basis cannot describe diffuse, loosely bound electrons, so the method says little about absolute electron affinities.
Comparing with Hartree-Fock and DFT values
Without electron correlation, Hartree-Fock virtual orbital energies tend to sit too high, and with diffuse basis functions the LUMO can become positive and lose physical meaning. In density functional theory, unoccupied orbital energies depend strongly on the chosen functional. Values from these methods, and from this calculator, belong to different scales. Compare within one method only.
Units and result interpretation
- Energies are reported in eV by default, with Hartree and kJ/mol available.
- Orbital energies are relative to an electron at rest at infinite separation, so bound orbitals are negative.
- A more negative LUMO indicates a stronger electron acceptor within a comparable series.
- Values are rounded for display. Downloads keep full double precision.
Factors that can influence results
The largest single source of difference between two reported values.
A different 3D structure gives a different orbital spectrum. This calculator uses one MMFF94s conformer from a fixed seed.
Wrong settings describe a different chemical species. Check the charge shown in the preview.
Gas-phase values differ from those in solvent or in a solid.
Calculations that fail are reported as errors, never as results.
Reporting a result
When you use a value from this site, report the method variant, basis set, geometry source and engine version shown in the results panel, so that others can reproduce it. The JSON download contains all of these together with the coordinates used. For legal terms of use, see the Disclaimer.
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