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Nexus: make tiling more robust #1432

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merged 4 commits into from Mar 12, 2019

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jtkrogel commented Mar 11, 2019

This PR fixes the issues raised in #1408. Full matrix tilings are trickier than first appreciated.

Nexus attempts to arrive at an effective diagonal tiling by reducing the tiling matrix into a volume preserving diagonal form via successive shear operations. This usually works, but the diagonal tiling apparently depends on the order of the shearing operations (e.g. you can sometimes get either (1,1,4) or (1,2,2)) as revealed by #1408.

This PR added tests of diagonal and matrix tiling and also a search over permutations in the shearing operation order. This change worked for the problem case of #1408, however it did not work for all of the randomly generated tiling matrices just added to the test set.

A final brute force fallback operation was added to catch the remaining cases: essentially find a bounding box for the tiled supercell in the basis of the untiled axes, generate new atomic coordinates in this bounding box, and then remove duplicate points. This approach was avoided originally because it's worst case scaling is order det(T)^3 rather than order det(T) as above.

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qmc-robot commented Mar 11, 2019

Can one of the maintainers verify this patch?

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Paul-St-Young commented Mar 11, 2019

This does resolve the problem case I found. Thanks @jtkrogel!

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markdewing commented Mar 11, 2019

okay to test

@ye-luo ye-luo merged commit 1f4ab09 into QMCPACK:develop Mar 12, 2019

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