+1 vote
asked Jun 30 by brendenroberts (220 points)

It appears that the ITensor svd operation does not ultimately call a LAPACK SVD routine. What advantage does this implementation have?

1 Answer

+1 vote
answered Jul 18 by miles (16,560 points)
selected Jul 19 by brendenroberts
Best answer

Hi Brendan,
Thanks for the question. This is the case mainly for historical reasons, because we were having some issues with the LAPACK SVD and needed a more reliable SVD as well as a more accurate one.

But it's been on my short list to put in a backend that calls LAPACK for cases where speed is the primary concern. Based on how the tests go, I may make LAPACK the default and allow the current implementation to be turned on with a flag for cases where high accuracy is needed (the current implementation uses a recursive algorithm that corrects errors made in the smallest singular values in earlier passes; it is described in the Appendix of the real-space parallel DMRG paper).


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