What is the CAR file format?
A .car file is a plain-text Cartesian coordinate file used in molecular modelling. Each file stores information about atomic positions - atom names, x/y/z coordinates, element symbols, residue identifiers, and partial charges - one record per atom.
.car files originated with the Biosym Insight II and Discover suite in the late 1980s and use a fully human-readable ASCII format. A .car file opens with the header line !BIOSYM archive 3, followed by a PBC=ON or PBC=OFF flag; when periodic boundary conditions are active, a unit-cell record listing cell dimensions and angles follows immediately. There are no binary magic bytes - identification relies on that header and the presence of an accompanying .mdf file (molecular definition), which holds topology, bond connectivity, and atom types.
.car files are normally distributed as a matched set: the coordinate archive (.car), a topology file (.mdf), and a force-field file (.frc) together fully define a simulation model. The same format is also written with the .arc extension. Ownership passed from Biosym Technologies through MSI (Molecular Simulations Inc.) and Accelrys to BIOVIA (Dassault Systèmes), which still reads and writes .car files via Materials Studio. Open-source converters such as Open Babel and Avogadro support the format across Windows, Linux, and macOS.
Security & safety
RISK: LOWA Biosym CAR file is harmless plain-text coordinate data - no executable content. The only real-world caution is identity confusion: a file labelled .car may actually be an Assetto Corsa model or a game archive (binary), not chemistry data. If a text editor shows "!BIOSYM archive 3" it is the coordinate format; if it shows binary garbage it is one of the other .car types.
Format details
in a nutshell- Assetto Corsa car model archive (.car) - Vehicle data/model package used by the Assetto Corsa racing-sim modding community; unrelated binary format.
- Generic packed archive (.car) - Various old archivers/games used .car as a bespoke compressed container (e.g. some EA titles); no single standard.
- CazaSoftware / 'Car Explorer' app file - App-specific .car referenced by the legacy DB entry 'Car Explorer'; obscure and not the chemistry coordinate format.
Programs that open CAR files
Windows4 apps
Avogadro
Open-source Free molecular editor/visualizer; open the .car to view atoms in 3D (via Open Babel backend). 
Open Babel
Open-source Read/convert the CAR with 'obabel input.car -O output.pdb'; the GUI build also opens it. 
BIOVIA Materials Studio
Paid Import the .car (with its .mdf) into the modelling environment to view/edit the structure. 
macOS1 app
Linux2 apps
Open Babel
Open-source Run 'obabel structure.car -O structure.pdb' to read or convert; install from your package manager. Technical details
deep spec| File class | Plain-text molecular Cartesian-coordinate archive (chemical modelling) |
| MIME type | chemical/x-msi-car |
| Header signature | ASCII text "!BIOSYM archive 3" at byte offset 0; no binary magic bytes |
| Periodic boundary flag | Second header line: PBC=ON (periodic crystal/polymer) or PBC=OFF (isolated molecule) |
| Unit-cell record | Present only when PBC=ON; specifies a, b, c lattice lengths and α, β, γ angles |
| Atom record fields | Atom name, x/y/z Cartesian coordinates (Å), residue name, element symbol, partial charge |
| Companion files | .mdf (topology and connectivity) and .frc (force-field parameters); .car alone is insufficient to fully define a model |
| Alternate extension | .arc - identical format; both .car and .arc are produced by Insight II and Materials Studio |
| Encoding | 7-bit ASCII plain text; no binary sections or byte-order dependency |
| Compression | None built-in; files are commonly distributed gzip-compressed (.car.gz) due to repetitive text |
| Typical file size | 1 KB to a few MB, scaling linearly with atom count |
| Format version token | "archive 3" embedded in the opening header; this revision has been stable since the early 1990s |
| Developer lineage | Biosym Technologies → MSI (Molecular Simulations Inc.) → Accelrys → BIOVIA (Dassault Systèmes) |
| Platform support | Windows, Linux, macOS - wherever BIOVIA Materials Studio, Open Babel, Avogadro, or VMD are installed |
| Released | Late 1980s-1990s (Biosym Insight II / Discover molecular modelling) |
| Latest version | Stable legacy format; still read/written by BIOVIA Materials Studio and Open Babel |
| Specification | openbabel.org |
CAR conversions
Community Q&A
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