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<div class="" style="margin: 0px; font-stretch: normal; line-height: normal;"><span class="" style="-webkit-font-kerning: none;">Dear Wannier90 users, contributors and developers,</span></div>
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<div class="" style="margin: 0px; font-stretch: normal; line-height: normal;"><span class="" style="-webkit-font-kerning: none;">I’m writing to update you on the ongoing development of Wannier90’s library interface. For those of you who were there, you may
recall that we first started discussions about this at the Developers’ Workshop in San Sebastian in 2016, and we have had follow-up discussions and further community-engagement since then. </span></div>
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<div class="" style="margin: 0px; font-stretch: normal; line-height: normal;"><span class="" style="-webkit-font-kerning: none;">Over the past year, in partnership with the UK's Computational Collaborative Project for the Study of the Electronic Structure of
Condensed Matter (CCP9), we[1] have started the process of designing and implementing the development of the library interface.</span></div>
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<div class="" style="margin: 0px; font-stretch: normal; line-height: normal;"><span class="" style="-webkit-font-kerning: none;">The preparatory work has necessitated some structural changes to the code, which are being done in a sequence of stages. There is
a wiki page at <a href="https://github.com/wannier-developers/wannier90/wiki/Code-restructure-(2021-2022)" class=""><span class="" style="-webkit-font-kerning: none; color: rgb(9, 79, 209);">https://github.com/wannier-developers/wannier90/wiki/Code-restructure-(2021-2022)</span></a> that
describes these stages and the expected timeline over which they will be merged into the “develop” branch of the Wannier90 GitHub repository, which is the branch from which code releases are periodically made. </span></div>
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<div class="" style="margin: 0px; font-stretch: normal; line-height: normal;"><span class="" style="-webkit-font-kerning: none;">The main take-home messages are:</span></div>
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<div class="" style="margin: 0px; font-stretch: normal; line-height: normal;"><span class="" style="-webkit-font-kerning: none;">+ for <i class="">users</i>, there is nothing to be concerned about: these changes do not affect the way that you use the code.</span></div>
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<div class="" style="margin: 0px; font-stretch: normal; line-height: normal;"><span class="" style="-webkit-font-kerning: none;">+ for <i class="">contributors and developers,</i> you will see that there is already a pull request for Stage 2 (PR 394) where
you can see what the next set of changes will be, and that there will soon be a pull request for Stage 3. Stage 2 will be merged into the develop branch in <i class="">approximately two weeks’ time</i>, and we expect Stage 3 will follow <i class="">by the
end of February, or early March. </i>New pull requests into the develop branch will not be accepted until after these mergers. This is because the restructure of the code is likely to result in merge conflicts if new contributed code is not reorganised following
the new structure and development guidelines. If you are currently working on code that you plan to contribute as a pull request in the future, we suggest that you check PR 394 (and the forthcoming PR for Stage 3, which will soon be available) and read the
wiki entry linked above, so you can start adapting your code.</span></div>
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<div class="" style="margin: 0px; font-stretch: normal; line-height: normal;"><span class="" style="-webkit-font-kerning: none;">If you have any questions, comments or suggestions, please feel free to email me.</span></div>
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<div style="margin: 0px; font-stretch: normal; line-height: normal;" class=""><span style="font-kerning: none" class="">Best wishes,</span></div>
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<div style="margin: 0px; font-stretch: normal; line-height: normal;" class=""><span style="font-kerning: none" class="">Arash</span></div>
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<div style="margin: 0px; font-stretch: normal; line-height: normal;" class="">[1] The developments are being implemented by Jerome Jackson, Barry Searle, Leon Petit and Martin Plummer of the UK’s Science and Technology Facilities Council, in collaboration with
the Wannier Developers Group.</div>
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<div>— <br class="">
Professor Arash Mostofi — <a href="http://www.mostofigroup.org" class="">www.mostofigroup.org</a><br class="">
Director, Thomas Young Centre @Imperial</div>
<div>Imperial College London</div>
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