Until now chemfp has only been available under the MIT license, which is a permissive free/open source software license.
This was true for both the 1.x development track, which is available for no cost, and the 3.x track, which was only available after paying for it.
Chemfp 1.x and 3.x are still available under the MIT license. In addition, for a lower price you can instead get chemfp 3.x with a license which doesn't let you redistribute the source code.
Several different licensing options are available, depending on your needs. Chemfp is available as a pre-compiled Python package for Linux, where some of the features require a time-based license key. This is mostly used for evaluation purposes.
Chemfp is also available with licensing options for academic use and for internal use at companies and other organizations. These licensees get a copy of the chemfp source code under a permanent license, along with the right to modify it. This is often called "source-available" license, to distinguish it from a "free software" or "open source" license.
For more details see the license page.
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