Non-contracting product in ITensor Julia

Question

Hi,

I know that the C++ ITensor versions overload the "/" operator to do non-contracting product between itensors. Is there a similar functionality in Julia? I couldn't find it very easily.

Thanks,
Amartya

Answer 1

Hi Amartya,
We haven’t defined that one in the Julia version. One reason is that it became hard to define it for QN conserving tensors in the C++ version. Another is just that we weren’t sure if anyone was using it.

Could I ask what your use case for it is?

Miles

Comments

Hi Miles,

Typically real time path integral codes are written using path arrays and other custom structures.  I am playing around with Feynman-Vernon influence functional formalism based on ITensor.  The first step was to have the dynamics of a single two level system coupled to a harmonic bath.  I am now thinking of doing many two level systems connected to each other as an extended system that is talking to a harmonic bath.  For a nearest neighbor interaction, it should be simple enough to do.  It would be a two-dimensional PEPS like decomposition.  I wanted to be able to handle non-nearest neighbor interactions by adding extra bonds instead of having the bond dimension grow out of bounds.  I think I might need something like a non-contracting product for that.

Using the SVD decomposition for reducing the size of the space and making the representation more compact might help a lot with extended systems with short range interactions.  I am already seeing some improvements over my old codes for the single two level system case in terms of storage requirements.  I have not yet compared performance, but those codes are in C++ and I was just playing with the Julia version, where my code may not be the most optimized.

I can discuss in greater depth if you are interested.

Best,

Amartya

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Hi Miles,

I have defined a non-contracting product of basic tensors which works for my case.  I don't know if it is of any interest to the community.  Would you please explain to me the issues arising with QN-conserving tensors?  I have never really used them in ITensor.

Hope you are having a great holiday!

Best,

Amartya