+1 vote
asked by (400 points)
edited by

From this answer

http://itensor.org/support/2799/are-there-methods-for-returning-value-for-mps-or-flux-for-mpo

I realized that one can use the "flux" function to get the quantum number flux of an ITensor. However I wonder if there is a method to get the quantum number flux for a particular element in an ITensor?

commented by (51.6k points)
Hi Chia-Min,
Is your original goal still the same? To generate a random ITensor with a certain flux? If so I can answer about that because we already have a function to do that.

Regarding your exact question above, there is not exactly a function to do what you are asking, the reason being that all non-zero elements of a QN ITensor are required by construction to have the same flux. So even if such a function existed, it would always give the same answer as calling `flux` on the whole ITensor. (Or it might throw an error if called on an element which is not allowed to be non-zero.)

Miles
commented by (51.6k points)
Based on the earlier version of  your question, I have added a new docs page finally documenting the version of `randomITensor` which accepts an QN argument, and makes a QN ITensor which has the requested flux and all elements set to random numbers which are compatible with the flux:

http://itensor.org/docs.cgi?vers=cppv3&page=classes/qnitensor
commented by (400 points)
Hi Miles. Thank you for the comments. Yes I still has that purpose, but I found that using the member function ".generate" with a random number generator seems will do the job and therefore I removed that part.

I have another reason for the flux function for a particular element but it is more complicated as follows. Given an MPS |psi> with no particle number QN, the overall goal is to 1) project |psi> to a particular particle number, and 2) cast the MPS to have particle-number QN. In the process, I need to cast an ITensor that does not have QN structure but should have a well defined QN, to an ITensor having QN structure. In this step I want to copy the elements based on checking the QN flux of the QN ITensor. I have implemented it by reading the QNs from the indices, but I think it would be very useful if there is a build-in function that gives the QN flux of a particular position. Since when assigning element to a QN ITensor, the code will check the flux for the proposed position, I thought there is already such a function there.

Of course if you have any comment about how to cast an MPS to having QN, it is also very helpful, but I think it would be kind of beyond this question.

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