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Yaff stands for "Yet another force field". It is a pythonic force-field code used by Toon and Louis to test-drive their new models. The original motivation to develop Yaff was to provide a good reference implementation of the force fields developed at the Center for Molecular Modeling (CMM) at Ghent University. In its current version, Yaff is general and flexible enough to handle a large variety of force field models.

More information about Yaff can be found on the CMM Code website:

Yaff is distributed as open source software under the conditions of the GPL license version 3. Read the file COPYING for more details, or visit


Yaff can be installed with pip (system wide or in a virtual environment):

pip install numpy Cython
pip install yaff

Alternatively, you can install Yaff in your home directory:

pip install numpy Cython --user
pip install yaff --user

Lastly, you can also install MolMod with conda. (See

conda install -c molmod yaff


The tests can be executed as follows:

nosetests yaff