  In order to allow different software applications, in constant evolution, to
interact and exchange data, flexible file formats are needed. A file format
specification for different types of content has been elaborated to allow
communication of data for the software developed within the European Network of
Excellence "NANOQUANTA", focusing on first-principles calculations of materials
and nanosystems. It might be used by other software as well, and is described
here in detail. The format relies on the NetCDF binary input/output library,
already used in many different scientific communities, that provides
flexibility as well as portability accross languages and platforms. Thanks to
NetCDF, the content can be accessed by keywords, ensuring the file format is
extensible and backward compatible.
