MOSAIC is a modular data model for molecular simulation applications pyMosaic is a Python library that provides an in-memory representation of MOSAIC data items plus input/output in HDF5 and XML formats. A command-line tool provides conversion between the HDF5 and XML representations as well as data import from the Protein Data Bank (PDB). This file is an ActivePapers edition of pyMosaic 0.4.0. ActivePapers is a computational science framework that supports reproducible research and publishing computations. An ActivePaper can contain any combination of code, data, and documentation. The pyMosaic ActivePaper is a code library with documentation. It can be referenced by other ActivePapers that contain molecular simulation data in Mosaic HDF5 format. For further information, see The pyMosaic Web site: http://pypi.python.org/pypi/pyMosaic The MOSAIC Web site: http://mosaic-data-model.github.io/ The ActivePapers Web site: http://www.activepapers.org/