First-principles electronic structure calculations are very widely used
thanks to the many successful software packages available. Their traditional
coding paradigm is monolithic, i.e., regardless of how modular its internal
structure may be, the code is built independently from others, from the
compiler up, with the exception of linear-algebra and message-passing
libraries. This model has been quite successful for decades. The rapid progress
in methodology, however, has resulted in an ever increasing complexity of those
programs, which implies a growing amount of replication in coding and in the
recurrent re-engineering needed to adapt to evolving hardware architecture. The
Electronic Structure Library (\esl) was initiated by CECAM (European Centre for
Atomic and Molecular Calculations) to catalyze a paradigm shift away from the
monolithic model and promote modularization, with the ambition to extract
common tasks from electronic structure programs and redesign them as free,
open-source libraries. They include ``heavy-duty'' ones with a high degree of
parallelisation, and potential for adaptation to novel hardware within them,
thereby separating the sophisticated computer science aspects of performance
optimization and re-engineering from the computational science done by
scientists when implementing new ideas. It is a community effort, undertaken by
developers of various successful codes, now facing the challenges arising in
the new model. This modular paradigm will improve overall coding efficiency and
enable specialists (computer scientists or computational scientists) to use
their skills more effectively. It will lead to a more sustainable and dynamic
evolution of software as well as lower barriers to entry for new developers.