Gaussian Formatted Checkpoint

Item	Value
Format ID	g16fchk
Extensions	.fchk, .fch, .fck
Read	Yes
Write	No
Registry role	Reader
Data level	Single-frame Gaussian QM results and coordinates

Quick read

Python

from molop import AutoParser

frame = AutoParser("molecule.fchk", parser_detection="g16fchk")[0][0]
print(len(frame.atoms), frame.coords.shape)
print(frame.energies.total_energy if frame.energies else None)
print(frame.charge_spin_populations.population_names if frame.charge_spin_populations else [])

Output contract

Output reports atom/coordinate size, available total energy, and population schemes actually present.

An fchk produces one current-geometry frame, not the full optimization trajectory from a log.

MolOP reads textual formatted checkpoints produced by Gaussian formchk. The reader decodes fixed-width records and N= array lengths without depending on adjacent field ordering. Unknown records are skipped.

Feature	Support	Scope	Limits
File recognition and fixture inventory	Fixture-covered	Reads fchk scalar and array records; every maintained fchk fixture, including large files, produces one result frame.	Binary .chk files are not accepted. Unknown records gain structured semantics only after an explicit shared-field mapping exists.
Metadata and route semantics	Partial	Parses title, Gaussian version, route, charge, multiplicity, model chemistry, tasks, dispersion, solvent, and open-shell treatment; the second-line summary fills method/basis gaps in the route grammar.	Route semantics remain bounded by the shared Gaussian grammar; fchk does not contain complete Link 0 input or runtime.
Geometry and Cartesian derivatives	Partial	Parses atomic numbers, Bohr coordinates, forces obtained by negating the Cartesian gradient, and a symmetric Hessian expanded from packed force constants.	An fchk represents one current geometry rather than a full optimization trajectory; coordinates and derivatives remain in source order without reconstructing the original input transform.
Energies, orbitals, and spin	Partial	Parses SCF/reference, MP2, MP3, MP4, CCSD, CCSD(T), and final total energies, alpha/beta orbital energies and occupancies, and S-squared.	MO coefficients, density matrices, natural orbitals, and method-specific energy labels outside the mapped set are not exposed.
Populations and electric response	Partial	Parses atom-count-aligned Mulliken, NPA, ESP, APT, Hirshfeld/CM5, and available spin population records, plus dipole, packed polarizability, and quadrupole values with atomic-unit conversion.	Only records present in the fchk are exposed; bond orders, hyperfine tensors, and higher electric-response arrays are not mapped.
Vibrations, thermochemistry, and status	Partial	Parses frequencies, reduced masses, force constants, IR intensities, normal modes, thermal energy/enthalpy/free energy, Job Status, and optimization completion.	Raman/ROA/VCD, detailed thermal corrections, temperature/pressure, and optimization convergence history are not mapped.
NMR shielding and spin-spin coupling	Partial	Parses per-atom shielding tensors and isotropic, anisotropic, and principal values; FC, SD, PSO, and DSO reduced-coupling matrices; and their total K matrix in Hz.	Requires the corresponding NMR records. fchk lacks the isotope information needed to reconstruct J; referenced chemical shifts and EPR data are not mapped.

Capability Coverage Matrix

fchk capability	Read	Structured result	Current boundary
Fixed-width scalar records	Supported	I, R, C, L, and H values	Unknown labels are skipped.
N= array records	Supported	Numeric values or 12-character chunks read to the declared length	Truncated arrays raise a format mismatch.
Title, route, and Gaussian version	Supported	title_card, keywords, qm_software_version	Original record layout is not retained.
Charge and multiplicity	Supported	charge, multiplicity	Uses the fchk scalar values.
Method, functional, basis, and tasks	Partial	model_chemistry, task_requests	Route takes precedence; the second-line summary fills missing method/basis data, while unknown route tokens remain diagnostic.
File and frame count	Supported	One segment and one terminal/SP frame	fchk does not represent the multi-Link or optimization-step timeline of a Gaussian log.
Atoms and current Cartesian coordinates	Supported	Angstrom coords, source-order atoms	Bohr coordinates are converted to Angstrom.
Cartesian gradient	Supported	Hartree/Bohr forces	Forces are the elementwise negative of the gradient.
Cartesian force constants	Supported	Symmetric (3N, 3N) hessian	Only a valid packed lower-triangle length is accepted.
SCF, MP2-4, CCSD, CCSD(T), and total energies	Supported	Energies and optional source evidence	Unmapped energy labels are not assigned guessed semantics.
Alpha/beta orbital energies and occupancies	Supported	MolecularOrbitals	If a closed-shell file omits beta energies, alpha energies are copied and beta occupancy uses the beta-electron count.
MO coefficients and density matrices	Unsupported	None	Large raw arrays are skipped safely.
Atomic population records	Partial	ChargeSpinPopulations, including extensible ESP/NPA spin series	Mulliken, NPA, ESP, APT, Hirshfeld/CM5, and known spin labels are mapped only when arrays match the atom count.
S-squared	Supported	TotalSpin	Spin quantum number is derived from S-squared.
Dipole, polarizability, and quadrupole	Partial	Polarizability	fchk packed ordering is retained; full named tensors are not expanded.
Frequencies, masses, force constants, and IR	Supported	Vibrations	Reads the known first four per-mode groups in Vib-E2.
Normal-mode displacements	Supported	One (N, 3) array per mode	Marked with source-program normalization and unknown mass weighting.
Thermal totals	Partial	U_T, H_T, G_T	Current fixtures do not contain full ZPVE, entropy, and heat-capacity decomposition.
NMR magnetic shielding	Partial	nmr.gauge, per-atom shielding_tensors, isotropic, anisotropy, and principal values	Reads NMR shielding; tensor units are converted to ppm and Cartesian orientation remains unknown. Per-atom shielding values can be projected into RDKit atom properties with qm_embedded_rdmol(embed_nmr=True).
NMR reduced spin-spin coupling	Partial	spin_spin_coupling_k, spin_spin_coupling_k_components, and coupling_atom_indices	Reads four packed FC/SD/PSO/DSO K contributions and sums them to total K in Hz; spin_spin_coupling_j and J contributions remain absent because fchk lacks isotope metadata; coupling matrices remain frame-level atom-pair data.
Job Status and optimization completion	Partial	Status, GeometryOptimizationStatus	Completion is inferred from an opt request plus normal Job Status; per-step convergence values are absent.
Source spans and hashes	Supported	One whole-file segment/frame provenance record	No .chk, log, or other sidecar is merged.
Basis, ECP, MO, and density raw arrays	Skipped	No dedicated shared fields	Being scannable does not imply structured semantic support.
fchk/chk writer	Unsupported	Reader only	MolOP does not rebuild fchk or process binary checkpoints.

Format Boundary

.fchk is a textual Gaussian formatted checkpoint, not a binary .chk. The reader does not invoke Gaussian formchk or load a neighboring log/checkpoint; all provenance describes only the supplied fchk text.