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.CAC file extension

To open .CAC files on Windows, confirm the file is intended as a chemical structure file (CAChe MolStruct) and not something from another unrelated app.

To open a .CAC file, treat it as a CAChe MolStruct chemical structure file and use chemistry software or a converter that supports “CAChe MolStruct”. If you do not have the original CAChe software, a practical approach is to convert it to a more widely supported chemical format using a tool such as Open Babel or an online converter that lists CAChe MolStruct.

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Last updated: April 29, 2026 · Reviewed by Julian Stricker

Open on your device

Choose your operating system for a dedicated step-by-step opening guide.

How to open .CAC files

Use these platform-specific instructions to open .CAC files safely.

Windows

  1. Confirm the file is intended as a chemical structure file (CAChe MolStruct) and not something from another unrelated app.
  2. Use a chemistry converter that explicitly supports “CAChe MolStruct format (.cac/.cache)” to convert the file to a common format you can view (for example, MOL or SDF).
  3. If you use Open Babel tools, be aware the CAChe MolStruct format is documented as write-only in Open Babel, so conversion may require a different tool if you need to read/import .CAC.
Full Windows guide

Mac

  1. If the file is CAChe MolStruct, use a chemistry conversion tool that lists support for “CAChe MolStruct format” to convert it to a more widely supported structure format.
  2. If an Open Babel-based app fails to import .CAC, check the Open Babel documentation note that the format is write-only and use an alternative converter instead.
Full Mac guide

Linux

  1. Check whether your workflow expects CAChe MolStruct (.cac/.cache) and the MIME type chemical/x-cache.
  2. Use a chemical format conversion tool that explicitly lists CAChe MolStruct support; if relying on Open Babel, note the documented write-only limitation for this format and choose a different tool if you need to read .CAC.
Full Linux guide

iOS

  1. iOS does not typically provide native support for CAChe MolStruct (.cac) files; transfer the file to a desktop system and convert it using a chemistry conversion tool that supports CAChe MolStruct.
Full iOS guide

Android

  1. Android typically has no common native support for CAChe MolStruct (.cac) files; move the file to a desktop system and convert it with a converter that lists CAChe MolStruct support.
Full Android guide

Security notes

  • .CAC (CAChe MolStruct) files are data files, but they are still parsed by complex chemistry libraries; only open/convert files from trusted sources to reduce the risk of parser vulnerabilities.
  • Avoid relying on automatic “MIME sniffing” from untrusted sources; ensure the file is actually a chemical structure file (commonly chemical/x-cache) before sending it into chemistry toolchains.
  • If a website offers to convert chemical files, consider the sensitivity of the chemical structure data you are uploading and whether you are allowed to share it externally.

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Can't open this file?

These are the most common causes and fixes when .CAC files fail to open.

Common reasons

  • No app will open the .CAC file
  • Open Babel (or an Open Babel-based app) cannot import the file
  • File type confusion: .CAC used for something else
  • Conversion produces incorrect or incomplete structures

Fix steps

  1. Verify it is a CAChe MolStruct file (often associated with chemical/x-cache and sometimes paired with the .cache extension).
  2. Use a converter that explicitly lists “CAChe MolStruct format” support to convert it to a widely supported chemical format.
  3. If you attempted to use Open Babel to read/import it, note that Open Babel documents CAChe MolStruct as write-only and try a different tool for reading.

OS-specific troubleshooting

What is a .CAC file?

.CAC (often also seen as .cache) is associated with the CAChe MolStruct chemical file format and is commonly mapped to the non-IANA MIME type chemical/x-cache. It stores molecular structure data for chemistry workflows. In Open Babel, this format is documented as write-only, which affects “opening” directly inside Open Babel-based apps.

Background

In practice, “.CAC” most often refers to the CAChe MolStruct format used for molecular structures in chemical informatics. Multiple references (Open Babel documentation and structured MIME/extension mappings) associate .cac/.cache with the MIME type chemical/x-cache.

This MIME type appears in real-world software ecosystems (for example, mime.types lists and MIME guessers) as chemical/x-cache, even though it is not an IANA-registered type. That makes it a common “best-effort” identification used by applications and servers when handling CAChe MolStruct files.

If you received a .CAC file and do not have the original CAChe environment, the most user-friendly path is typically conversion to a more common chemistry format (e.g., MOL/SDF) using chemistry conversion tools that explicitly list CAChe MolStruct as an input/output option. Note that Open Babel’s documentation indicates CAChe MolStruct support is write-only there, so direct import/viewing via Open Babel may not work as expected even if the format is recognized.

Common MIME types: chemical/x-cache

Further reading

Authoritative resources for more details on the .CAC format.

Common .CAC issues

No app will open the .CAC file

Many systems do not ship with chemical-structure tooling, and .CAC is not a general-purpose format. Also, some toolchains may recognize the type but not actually support reading it.

  1. Verify it is a CAChe MolStruct file (often associated with chemical/x-cache and sometimes paired with the .cache extension).
  2. Use a converter that explicitly lists “CAChe MolStruct format” support to convert it to a widely supported chemical format.
  3. If you attempted to use Open Babel to read/import it, note that Open Babel documents CAChe MolStruct as write-only and try a different tool for reading.

Open Babel (or an Open Babel-based app) cannot import the file

Open Babel documentation states CAChe MolStruct is write-only, so tools built on Open Babel may not be able to open/read .CAC files.

  1. Check the Open Babel CAChe MolStruct format documentation for the read/write limitation.
  2. Use a different converter or the originating CAChe workflow/application to export to another format, then open the exported file.

File type confusion: .CAC used for something else

The .cac extension can be ambiguous across the broader software world; opening it with the wrong kind of program will fail.

  1. Ask the sender what application produced the file and what it contains (chemical structure vs. unrelated data).
  2. Check whether your environment identifies it as chemical/x-cache; if not, treat it as an unknown binary/data file and obtain the correct originating software.

Conversion produces incorrect or incomplete structures

Chemical structure formats can differ in what metadata or structure features they store; conversions may lose information or map fields differently.

  1. After conversion, validate the resulting structure in your chemistry workflow (atoms, bonds, charges, stereochemistry).
  2. Try converting with a different converter/tool if the results look wrong, and compare outputs.

FAQ

What does a .CAC file usually contain?

Most commonly, it is a CAChe MolStruct chemical structure file (also seen with the .cache extension) associated with the MIME type chemical/x-cache.

Why won’t Open Babel open my .CAC file?

Open Babel’s documentation for the CAChe MolStruct format notes it is write-only, so Open Babel-based tools may not support reading/importing .CAC even if they can export to it.

Is the MIME type for .CAC officially registered?

The common mapping is chemical/x-cache, which is used in practice in software ecosystems and mime.types lists, but it is part of the non-IANA “chemical/x-*” style of MIME types used historically for chemical formats.

Can I fix the file by renaming .CAC to another extension?

No. Renaming only changes the extension label; you typically need to convert/export the file using a tool that understands CAChe MolStruct.

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