Core terminology
Look up terms you see in commands and results. Each definition explains how openWFN uses the term.
Calculation
The data openWFN reads from one input file: structure, calculation details, available wavefunction records and information about the source.
Formatted checkpoint (.fchk)
A text representation produced by Gaussian's formchk utility. openWFN reads this
format directly. A binary .chk file is proprietary and must be converted externally.
Atomic unit
A natural unit system used in electronic-structure theory. Energies are commonly
reported in hartree (E_h or a.u.); coordinates in checkpoint records may be stored
in bohr and converted to ångströms at the parser boundary.
Basis function
A mathematical function used to expand molecular orbitals. openWFN evaluates the supported contracted Cartesian Gaussian basis representation when required records are present.
Molecular orbital
A linear combination of basis functions described by coefficients, energy, and occupation. The highest occupied and lowest unoccupied orbitals are called HOMO and LUMO.
Density matrix
A matrix representation used with basis functions to evaluate electron density and population analyses. Total and spin density matrices can determine alpha and beta components when the required records are available.
Population analysis
A scheme for partitioning electrons among atoms. Mulliken and symmetric Löwdin methods are model-dependent; their charges are not directly observable quantities.
Electrostatic potential (ESP)
Potential at a point arising from nuclei, an atomic-charge model, electrons, or the combined system. These variants are not interchangeable and have different status.
Provenance
Information connecting a result to its input bytes, parser, software version, parameters, method, and capability status.
Capability status
File capabilities show which data and analyses are available. The separate validation labels Stable, Validated, Experimental and Unsupported describe interface support and scientific evidence. See Validation.