The research and development of maximally-localised Wannier functions [REF] started with a National Science Foundation CISE Postdoctoral Fellowship for Nicola Marzari to work with David Vanderbilt at Rutgers University in 1996-98. The original Fortran77 code was restructured on Thanksgiving Day 1996, and hence for many years known as turkey.f. In this incarnation, it was interfaced directly to the all-bands ensemble-DFT CASTEP code [REF] of the Cambridge-Keele collaboration.

In those early days, maximally-localised Wannier functions were used by the American Physical Society in the announcement of Walter Kohn’s Nobel Prize for the development of density-functional theory (see here and here).

In 1998 the algorithm was implemented in the CPMD code by Pierluigi Silvestrelli, in the group of Michele Parrinello at MPI-Stuttgart [REF]. Public availability spurred its early popularity, leading to many applications in which Wannier functions were used to study the electronic-structure of complex systems, thanks to their connection with our intuitive understanding of chemical bonding. There is also a formal connection to the dielectric theory of solids, via the Berry phase formulation of macroscopic polarisation (King-Smith and Vanderbilt [REF], Resta [REF]), and Wannier functions centers represent a mapping onto classical charges of quantum dielectric properties [REF].

Ivo Souza (then a PhD student with Richard Martin, UIUC) became involved in the project in 1999, first in an application to compressed hydrogen, and then during his postdoctoral days at Rutgers, leading to the Souza-Marzari-Vanderbilt disentanglement extension [REF].

Several visits of Nicola Marzari to Michel Posternak and Alfonso Baldereschi, at the Ecole Polytechnique Federale de Lausanne, brought the interface to FLAPW and to any generic electronic-structure code [REF].

Yudong Wu and Manu Sharma (working with Nicola Marzari and Roberto Car at Princeton University) implemented the algorithm in CP90/Quantum ESPRESSO, Francois Gygi (LLNL) in JEEP/QBox [REF], Leonardo Bernasconi (University of Oxford) in CASTEP.

A collaboration with Marco Buongiorno Nardelli (NCSU) led to the Landauer transport applications, in collaboration with Arrigo Calzolari and Andrea Ferretti at the University of Modena (see here) [REF] and Young-Su Lee at MIT [REF]. Thygesen and Jacobsen (CAMP, Copenhagen) have led analogous developments centered around the DACAPO code.

In 2006 the code was rewritten in Fortran90 – restructured, extended and optimised – by Arash Mostofi and Jonathan Yates, Research Fellows at the University of Cambridge, and postdoctoral researchers in the groups of Nicola Marzari and Ivo Souza, respectively. Stefano de Gironcoli and Malgorzata Wierzbowska interfaced Wannier90 to operate seamlessly with Quantum ESPRESSO. Interfaces to many other electronic structure codes were written over the following years.

The v2.0 release of Wannier90 in 2013 added a large amount of new functionality built around the postw90.x executable. This performs Wannier interpolation using MPI parallelisation. In particular Boltzmann transport functionality was added by Giovanni Pizzi at EPFL, while Ivo Souza added Berry phase related properties such as the anomalous Hall conductivity and orbital magnetisation.

The formalism has seen many and diverse applications: linear-scaling quantum Monte-Carlo (Williamson, Hood and Grossman, LLNL), photonic crystals (Whittaker and Croucher, Garcia-Martin and Wolfle), metal-insulator interfaces (Stengel and Spaldin), and as an efficient interpolator for the anomalous Hall effect (Wang, Yates, Souza and Vanderbilt) and electron-phonon couplings (Giustino, Yates and Souza), to cite only a few.

In addition, Wannier functions are playing an increasing role in bridging density-functional approaches and strongly-correlated ones, to derive model Hamiltonians or as a starting point for LDA+U or LDA+DMFT (Ku and Pickett, Georges, Vollhardt, Solovyev, Anisimov, and many others). They are also closely related to the order-N muffin-tin orbitals (NMTOs) developed by O. K. Andersen and collaborators (MPI-Stuttgart).

In 2012 a comprehensive review article on Wannier functions was published in Reviews of Modern Physics [REF].

In 2016 a developer workshop was held in San Sebastian. This coincided with the move of the Wannier90 repository to GitHub to enable easier integration of community contributions. One of the first such contributions was the ability to compute symmetry-adapted Wannier functions [REF].

2019 saw the first major release of the code (v3.0.0) since the transition to a more community-driven development model, including many new items of functionality contributed by the wider ensemble of Wannier90 users and developers, and described in detail in a new community-authored paper that appeared on J. Phys. Cond. Matt. in early 2020 [REF], with an update in March 2020 with v3.1 of the code.

Reflecting the increasing impact of the diverse ecosystem of Wannier-related software, the 2022 edition of the Wannier Developers Meeting, held at the ICTP in Trieste, was devoted to fostering cooperation and integration between Wannier-related software packages. This meeting was immediately preceded by a successful and very well attended Wannier Summer School. The theme of developing and supporting the wider ecosystem of Wannier-related codes was continued at the 2023 Wannier Developers Meeting, held at the Daresbury Laboratory (Warrington, UK). A review paper describing the Wannier software ecosystem is currently under review, with a pre-print available on arXiv.

A Wannier Developers Meeting was held in February 2024 at the Paul Scherrer Institute in Switzerland. One of the key focusses of this meeting was preparation of the Wannier90 code for its forthcoming next major release (v4.0). This release will feature a library interface to Wannier90 functionality that is capable of being invoked by an external calling program in a parallel (MPI) environment, with significant changes to the structure of the code since the last release (v3.1.0). The development of the library has been part of the UK’s Computational Collaborative Project for the Study of the Electronic Structure of Condensed Matter (CCP9) and in collaboration with Jerome Jackson, Barry Searle and Leon Petit from the Scientific Computing Division of the UK Science and Technology Facilities Council (STFC).