Anatomy of a formatted checkpoint file
Use this page when your calculation program produces Gaussian checkpoint
files. They are one of several input families supported by openWFN. A .chk file is binary. Gaussian's formchk converts it
to a text .fchk file that openWFN can read directly. openWFN can call an
installed formchk, but cannot decode the binary format itself.
Record structure
An FCHK file begins with descriptive header lines and then labeled scalar or array records. A record identifies its data type and, for arrays, a count. Common groups include:
| Group | Examples | Used for |
|---|---|---|
| Molecular state | charge, multiplicity, electron counts | summary and spin interpretation |
| Structure | atomic numbers, Cartesian coordinates | geometry and topology |
| Basis | shell types, exponents, contraction coefficients | Gaussian basis evaluation |
| Orbitals | orbital energies and MO coefficients | frontier-orbital analysis |
| Density | total and spin density matrices | density, population, and ESP methods |
| Calculation metadata | total energy, method-dependent records | provenance and diagnostics |
Not every job writes every record. A geometry-only file may support distances but not density analysis. Restricted and unrestricted calculations can also expose different orbital and density records.
Coordinates and units
FCHK Cartesian coordinates are stored in atomic units. openWFN converts molecular coordinates to ångströms at the parser boundary for structural operations and presentation. Grid controls for density and ESP use bohr, as stated in the CLI help and method pages.
Conversion
This requires Gaussian's formchk in PATH. If you do not have Gaussian utilities, request an .fchk file from the calculation author.
Inspect before analysis
Treat FCHK files as research data: they may encode unpublished structures and results. See Security and data privacy.