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First-principles electronic structure calculations are now accessible to a very large community of users across many disciplines, thanks to many successful software packages, some of which are described in this special issue. The traditional coding paradigm for such packages is monolithic, i.e., regardless of how modular its internal structure may be, the code is built independently from others, essentially from the compiler up, possibly with the exception of linear-algebra and message-passing libraries. This model has endured and been quite successful for decades. The successful evolution of the electronic structure methodology itself, however, has resulted in an increasing complexity and an ever longer list of features expected within all software packages, which implies a growing amount of replication between different packages, not only in the initial coding but, more importantly, every time a code needs to be re-engineered to adapt to the evolution of computer hardware architecture. The Electronic Structure Library (ESL) was initiated by CECAM (the 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 codes and redesign them as open-source libraries available to everybody. Such libraries include "heavy-duty" ones that have the potential for a high degree of parallelization and 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, e.g., physicists and chemists when implementing new ideas. We envisage that this modular paradigm will improve overall coding efficiency and enable specialists (whether they be computer scientists or computational scientists) to use their skills more effectively and will lead to a more dynamic evolution of software in the community as well as lowe

First-principles electronic structure calculations are now accessible to a very large community of users across many disciplines, thanks to many successful software packages, some of which are described in this special issue. The traditional coding paradigm for such packages is monolithic, i.e., regardless of how modular its internal structure may be, the code is built independently from others, essentially from the compiler up, possibly with the exception of linear-algebra and message-passing libraries. This model has endured and been quite successful for decades. The successful evolution of the electronic structure methodology itself, however, has resulted in an increasing complexity and an ever longer list of features expected within all software packages, which implies a growing amount of replication between different packages, not only in the initial coding but, more importantly, every time a code needs to be re-engineered to adapt to the evolution of computer hardware architecture. The Electronic Structure Library (ESL) was initiated by CECAM (the 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 codes and redesign them as open-source libraries available to everybody. Such libraries include "heavy-duty" ones that have the potential for a high degree of parallelization and 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, e.g., physicists and chemists when implementing new ideas. We envisage that this modular paradigm will improve overall coding efficiency and enable specialists (whether they be computer scientists or computational scientists) to use their skills more effectively and will lead to a more dynamic evolution of software in the community as well as l

First-principles electronic structure calculations are now accessible to a very large community of users across many disciplines, thanks to many successful software packages, some of which are described in this special issue. The traditional coding paradigm for such packages is monolithic, i.e., regardless of how modular its internal structure may be, the code is built independently from others, essentially from the compiler up, possibly with the exception of linear-algebra and message-passing libraries. This model has endured and been quite successful for decades. The successful evolution of the electronic structure methodology itself, however, has resulted in an increasing complexity and an ever longer list of features expected within all software packages, which implies a growing amount of replication between different packages, not only in the initial coding but, more importantly, every time a code needs to be re-engineered to adapt to the evolution of computer hardware architecture. The Electronic Structure Library (ESL) was initiated by CECAM (the 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 codes and redesign them as open-source libraries available to everybody. Such libraries include “heavy-duty” ones that have the potential for a high degree of parallelization and 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, e.g., physicists and chemists when implementing new ideas. We envisage that this modular paradigm will improve overall coding efficiency and enable specialists (whether they be computer scientists or computational scientists) to use their skills more effectively and will lead to a more dynamic evolution of software in the community as well as lowe, The authors would like to thank CECAM for launching and pushing the ESL, as well as hosting part of its infrastructure, and partly funding the extended workshops where most of the coding was done, both in the Lausanne headquarters and in the Dublin, Trieste, and Zaragoza nodes. Within CECAM, the authors particularly thank Sara Bonella, Bogdan Nichita, and Ignacio Pagonabarraga. The authors also acknowledge all the people who have supported and contributed to the ESL in different ways, including Luis Agapito, Xavier Andrade, Balint Aradi, Emanuele Bosoni, Lori A. Burns, Christian Carbogno, Ivan Carnimeo, Abel Carreras Conill, Alberto Castro, Michele Ceriotti, Anoop Chandran, Wibe de Jong, Pietro Delugas, Thierry Deutsch, Hubert Ebert, Aleksandr Fonari, Luca Ghiringhelli, Paolo Giannozzi, Matteo Giantomassi, Judit Gimenez, Ivan Girotto, Xavier Gonze, Benjamin Hourahine, Jürg Hutter, Thomas Keal, Jan Kloppenburg, Hyungjun Lee, Liang Liang, Lin Lin, Jianfeng Lu, Nicola Marzari, Donal MacKernan, Layla Martin-Samos, Paolo Medeiros, Fawzi Mohamed, Jens Jørgen Mortensen, Sebastian Ohlmann, David O’Regan, Charles Patterson, Etienne Plésiat, Markus Rampp, Laura Ratcliff, Stefano Sanvito, Paul Saxe, Matthias Scheffler, Didier Sebilleau, Søren Smidstrup, James Spencer, Atsushi Togo, Joost Vandevondele, Matthieu Verstraete, and Brian Wylie. The authors would also like to thank the Psi-k network for having partially funded several of the ESL workshops. A.O., E.A., D.L.-D., S.G., E.K., A.A.M., and M.C.P. received funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 676531 (Centre of Excellence project E-CAM). The same project has partly funded the extended software development workshops in which most of the ESL coding effort has happened. A.G., S.M., and E.A. acknowledge support from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 824143 (Centre of Excellence project MaX). M.A.L.M. ackn, Peer Reviewed, Postprint (author's final draft)

This review presents the state of the art in molecular simulations of interfacial systems and of the calculation of the surface tension from the underlying intermolecular potential. We provide a short account of different methodological factors (size-effects, truncation procedures, long-range corrections and potential models) that can affect the results of the simulations. Accurate calculations are presented for the calculation of the surface tension as a function of the temperature, pressure and composition by considering the planar gas-liquid interface of a range of molecular fluids. In particular, we consider the challenging problems of reproducing the interfacial tension of salt solutions as a function of the salt molality; the simulations of spherical interfaces including the calculation of the sign and size of the Tolman length for a spherical droplet; the use of coarse-grained models in the calculation of the interfacial tension of liquid-liquid surfaces and the mesoscopic simulations of oil-water-surfactant interfacial systems.