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In the coupled cluster code of PySCF, is the four index tensor stored in memory or in disk?

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    $\begingroup$ A good place to start would be the docs or even the source code itself. Note, there are multiple implementations of CC in PySCF, some of which explicitly mention using disk. $\endgroup$
    – Tyberius
    May 19, 2022 at 17:41
  • $\begingroup$ Usually data is not stored on disk by default, since this would make things very slow. Likely the 4-index tensor is stored in RAM by default, and when it is expected to take up too much space to fit in RAM, the code adapts itself to store the data on disk. If you run a tiny CC calculation (minimal basis set and just a few electrons), do you see any files getting written to disk? $\endgroup$ May 19, 2022 at 20:41
  • $\begingroup$ Thanks for the responds. I have views PySCF’s source code and docs. In their documentation, they claim that for rccsd method, they claims that everything is stored on disk but their CC residue code, they use einsum a lot and many of them has four index tensors as input. I just don’t understand how could they use einsum to do tensor products on 4-index tensors while still store them on disk. $\endgroup$
    – Paulie Bao
    May 20, 2022 at 2:09
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    $\begingroup$ Typically CC codes are based on block tensors, since the whole tensors are so large that they do not fit into memory all at once. Instead, one handles the tensors one block at a time, enabling the efficient use of einsum operations, for instance. $\endgroup$ May 27, 2022 at 19:12
  • $\begingroup$ Hi Paulie! I gave my +1 long ago, but I wonder whether or not you figured out the answer to this question over the last ~7 months? $\endgroup$ Dec 10, 2022 at 16:11

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