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Specification of an extensible and portable file format for electronic structure and crystallographic data

X. Gonzea, C.-O. Almbladh, A. Cucca, D. Caliste, C. Freysoldt, M.A.L. Marques, V. Olevano, Y. Pouillon and M.J. Verstraet

Comput. Mater. Sci. (2008) doi:10.1016/j.commatsci.2008.02.023


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 across languages and platforms. Thanks to NetCDF, the content can be accessed by keywords, ensuring the file format is extensible and backward compatible.


Electronic structure; File format standardization; Crystallographic datafiles; Density datafiles; Wavefunctions datafiles; NetCDF

PACS classification codes

61.68.+n; 63.20.dk; 71.15.Ap

Problems with the binary representations:

  1. lack of portability between big-endian and little-endian platforms (and vice-versa), or between 32-bit and 64-bit platforms;
  2. difficulties to read the files written by F77/90 codes from C/C++ software (and vice-versa)
  3. lack of extensibility, as one file produced for one version of the software might not be readable by a past/forthcoming version

(Network Common Data Form) library solves these issues. (Alas, it is binary and in the examples presented in  http://www.unidata.ucar.edu/software/netcdf/examples/files.html, it seems like the CDF representations are larger in size than those of the CDL metadata representations)

The idea of standardization of file formats is not new in the electronic structure community [X. Gonze, G. Zerah, K.W. Jakobsen, and K. Hinsen. Psi-k Newsletter 55, February 2003, pp. 129-134. URL: <http://psi-k.dl.ac.uk>] (This article is a must read article, by the way. It’s a very insightful and concerning overview of the good natured developments in the atomistic/molecular software domain. It is purifying in the attempt to produce something good..). However, it proved difficult to achieve without a formal organization, gathering code developers involved in different software project, with a sufficient incentive to realize effective file exchange between such software.

HDF (Hierarchical Data Format) is also an alternative. NetCDF is simpler to use if the data formats are flat, while HDF has definite advantages if one is dealing with hierarchical formats. Typically, we will need to describe many different multi-dimensional arrays of real or complex numbers, for which NetCDF is an adequate tool.

Although a data specification might be presented irrespective of the library actually used for the implementation, such a freedom might lead to implementations using incompatible file formats (like NetCDF and XML for instance). This possibility would in effect block the expected standardization gain. Thus, as a part of the standardization, we require the future implementations of our specification to rely on the NetCDF library only. (Alternative to independent schemas is presented as a mandatory format which is against the whole idea of development. The reason for NetCDF being incompatible with XML lies solely on the inflexibility/inextensibility of the prior format. Although such a readily defined and built format is adventageous considering the huge numerical data and parameters the ab inito software uses, it is very immobile and unnecessary for compound and element data.)

The compact representation brought by NetCDF can be by-passed in favour of text encoding (very agreable given the usage purposes: XML type extensible schema usage is much more adequate for structure/composition properties). We are aware of several standardization efforts [J. Junquera, M. Verstraete, X. Gonze, unpublished] and [J. Mortensen, F. Jollet, unpublished. Also check Minutes of the discussion on XML format for PAW setups], relying on XML, which emphasize addressing by content to represent such atomic data.

4 types of data types are distinguished:

  • (A) The actual numerical data (which defines whether a file contains wavefunctions, a density, etc), for which a name must have been agreed in the specification.
  • (B) The auxiliary data that is mandatory to make proper usage of the actual numerical data of A-type. The name and description of this auxiliary information is also agreed.
  • (C) The auxiliary data that is not mandatory to make proper usage of the A-type numerical data, but for which a name and description has been agreed in the specification.
  • (D) Other data, typically code-dependent, whose availability might help the use of the file for a specific code. The name of these variables should be different from the names chosen for agreed variables of A–C types. Such type D data might even be redundant with type A–C data.

The NetCDF interface adapts the dimension ordering to the programming language used. The notation here is C-like, i.e. row-major storage, the last index varying the fastest. In FORTRAN, a similar memory mapping is obtained by reversing the order of the indices. (So, the ordering/reverse-ordering is handled by the interface/library)

Concluding Remarks

We presented the specifications for a file format relying on the NetCDF I/O library with a content related to electronic structure and crystallographic data. This specification takes advantage of all the interesting properties of NetCDF-based files, in particular portability and extensibility. It is designed for both serial and distributed usage, although the latter characteristics was not presented here.

Several software in the Nanoquanta and ETSF [15] context can produce or read this file format: ABINIT [16] and [17], DP [18], GWST, SELF [19], V_Sim [20]. In order to further encourage its use, a library of Fortran routines [5] has been set up, and is available under the GNU LGPL licence.

Additional Information:

Announcement for a past event
( http://www.tddft.org/pipermail/fsatom/2003-February/000004.html )

CECAM – psi-k – SIMU joint Tutorial

1) Title : Software solutions for data exchange and code gluing.

Location : Lyon Dates : 8-10 october, 2003

Purpose : In this tutorial, we will teach software tools and standards that have recently emerged in view of the exchange of data (text and binary) and gluing of codes : (1) Python, as scripting language, its interfaces with C and FORTRAN ; (2) XML, a standard for representing structured data in text files ; (3) netCDF, a library and file format for the exchange and storage of binary data, and its interfaces with C, Fortran, and Python

Organizers : X. Gonze gonze@pcpm.ucl.ac.be

K. Hinsen hinsen@cnrs-orleans.fr

2) Scientific content

Recent discussions, related to the CECAM workshop on "Open Source Software for Microscopic Simulations", June 19-21, 2002, to the GRID concept (http://www.gridcomputing.com), as well as to the future Integrated Infrastructure Initiative proposal linked to the European psi-k network (http://psi-k.dl.ac.uk), have made clear that one challenge for the coming years is the ability to establish standards for accessing codes, transferring data between codes, testing codes against each other, and become able to "glue" them (this being facilitated by the Free Software concept).

In the present tutorial, we would like to teach three "software solutions" to face this challenge : Python, XML and netCDF.

Python is now the de facto "scripting langage" standard in the computational physics and chemistry community. XML (eXtended Markup Language) is a framework for building mark-up languages, allowing to set-up self-describing documents, readable by humans and machines. netCDF allows binary files to be portable accross platforms. It is not our aim to cover all possible solutions to the above-mentioned challenges (e.g. PERL, Tcl, or HDF), but these three have proven suitable for atomic-scale simulations, in the framework of leading projects like CAMPOS (http://www.fysik.dtu.dk/campos), MMTK (http://dirac.cnrs-orleans.fr/MMTK), and GROMACS (http://www.gromacs.org). Other software projects like ABINIT (http://www.abinit.org) and PWSCF (http://www.pwscf.org – in the DEMOCRITOS context), among others, have made clear their interest for these. All of these software solutions can be used without having to buy a licence.

Tentative program of the tutorial. Lectures in the morning, hands-on training in the afternoon.

1st day ——- 2h Python basics 1h Interface : Python/C or FORTRAN 1h XML basics Afternoon Training with Python, and interfaces with C and FORTRAN

2nd day ——- 2h Python : object oriented (+ an application to GUI and Tk) 1h Interface : Python/XML 1h Interface : XML + C or FORTRAN Afternoon Training with XML + interfaces

3rd day ——- 1h Python : numerical 1h netCDF basics 1h Interface : netCDF/Python 1h Interface : netCDF/C or FORTRAN Afternoon Training with netCDF + interfaces

3) List of lecturers

K. Hinsen (Orleans, France), organizer X. Gonze (Louvain-la-Neuve, Belgium), organizer K. Jakobsen (Lyngby, Denmark), instructor J. Schiotz (Lyngby, Denmark), instructor J. Van Der Spoel (Groningen, The Netherlands), instructor M. van Loewis (Berlin, Germany), instructor

4) Number of participants : around 20, Most of the participants should be PhD students, postdoc or young permanent scientists, involved in code development. It is assumed that the attendants have a good knowledge of UNIX, and C or FORTRAN.

Our budget will allow contributing to travel and local expenses of up to 20 participants.

XML and NetCDF:

from Specification of file formats for NANOQUANTA/ETSF Specification – www.etsf.eu/research/software/nq_specff_v2.1_final.pdf

Section 1. General considerations concerning the present file format specifications.

One has to consider separately the set of data to be included in each of different types of files, from their representation. Concerning the latter, one encounters simple text files, binary files, XML-structured files, NetCDF files, etc … It was already decided previously (Nanoquanta meeting Maratea Sept. 2004) to evolve towards formats that deal appropriately with the self- description issue, i.e. XML and NetCDF. The inherent flexibility of these representations will also allow to evolve specific versions of each type of files progressively, and refine earlier working proposals. The same direction has been adopted by several groups of code developers that we know of.

Information on NetCDF and XML can be obtained from the official Web sites,

http://www.unidata.ucar.edu/software/netcdf/ and


There are numerous other presentations of these formats on the Web, or in books.

The elaboration of file formats based on NetCDF has advanced a lot during the Louvain-la- Neuve mini-workshop. There has been also some remarks about XML.

Concerning XML :

(A) The XML format is most adapted for the structured representation of relatively small quantity of data, as it is not compressed.

(B) It is a very flexible format, but hard to read in Fortran (no problem in C, C++ or Python). Recently, Alberto Garcia has set up a XMLF90 library of routines to read XML from Fortran. http://lcdx00.wm.lc.ehu.es/~wdpgaara/xml/index.html Other efforts exists in this direction http://nn-online.org/code/xml/

Concerning NetCDF

  • (A) Several groups of developers inside NQ have already a good experience of using it, for the representation of binary data (large files).
  • (B) Although there is no clear advantage of NetCDF compared to HDF (another possibility for large binary files), this experience inside the NQ network is the main reason for preferring it. By the way, NetCDF and HDF are willing to merge (this might take a few years, though).
  • (C) File size limitations of NetCDF exist, see appendix D, but should be overcome in the future.

Thanks to the flexibility of NetCDF, the content of a NetCDF file format suitable for use for NQ softwares might be of four different types :

(1) The actual numerical data (that defines a file for wavefunctions, or a density file, etc …), whose NetCDF description would have been agreed.

(2) The auxiliary data that are mandatory to make proper usage of the actual numerical data. The NetCDF description of these auxiliary data should also be agreed.

(3) The auxiliary data that are not mandatory, but whose NetCDF description has been agreed, in a larger context.

(4) Other data, typically code-dependent, whose existence might help the use of the file for a specific code.

[5] URL: <http://www.etsf.eu/index.php?page=tools>.

[15] URL: <http://www.etsf.eu/>.

[16] X. Gonze, J.-M. Beuken, R. Caracas, F. Detraux, M. Fuchs, G.-M. Rignanese, L. Sindic, M. Verstraete, G. Zerah, F. Jollet, M. Torrent, A. Roy, M. Mikami, Ph. Ghosez, J.-Y. Raty and D.C. Allan, Comput. Mater. Sci. 25 (2002), pp. 478–492.

[17] X. Gonze, G.-M. Rignanese, M. Verstraete, J.-M. Beuken, Y. Pouillon, R. Caracas, F. Jollet, M. Torrent, G. Zérah, M. Mikami, Ph. Ghosez, M. Veithen, J.-Y. Raty, V. Olevano, F. Bruneval, L. Reining, R. Godby, G. Onida, D.R. Hamann and D.C. Allan, Zeit. Kristall. 220 (2005), pp. 558–562.

[18] URL: <http://dp-code.org>.

[19] URL: <http://www.bethe-salpeter.org>.

[20] URL: http://www-drfmc.cea.fr/sp2m/L_Sim/V_Sim/index.en.html.

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