define initial state in iTEBD

Question

Dear itensor developer ,

The questions are related to https://github.com/hoihui/itebd

How one can define an initial state in iTEBD?
For example, in https://github.com/hoihui/itebd/blob/master/1503.02010.cpp
and what does mean the quantum numbers Qa=2 and Qb=3: Sys(ampo,2,3); in this code.

'Sys.step_imag(dt, 20, 0, M)' is designed to obtain the ground energy or ground state?

Thank you in advance.

Answer 1

Hi, so unfortunately we can't provide official user support specifically for codes listed on the "ITensor codes" page. This is because those codes are written by a variety of people outside the ITensor development team, and we do not always understand their inner workings in detail.

But taking a quick look at that code, it appears that QNA and QNB are how you set an initial state. I believe that setting these creates an initial state which is a product state where each site (of the two sites) has a state defined by the quantum numbers QNA, and QNB, where these quantum numbers are defined through the IQIndex sites (but unfortunately the description of this part is vague, I agree, so you'll have to read the code or ask the code's developer).

He does describe the usage somewhat in the readme file here: https://github.com/hoihui/itebd

The line "Sys(ampo,2,3)" is defining an object of type "itebd" and calling its constructor function, which takes an AutoMPO object and two integers. The itebd type is defined as "struct itebd ..." in the itebd.h file.

Best regards,
Miles

Comments

Anybody knows hoihui's e-mail https://github.com/hoihui?
He doesnt answer at githhub?

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Hi, I am the writer of that code and just found this.
The numbers 2, 3 in "Sys(ampo,2,3)" will be used to initialize the "SiteSet" class of each of the two sites. For example, in that code the Sites are "Hubbard", so the reference is http://itensor.org/docs.cgi?vers=cppv2&page=classes/hubbard

I do not remember why I pass numbers instead of strings for the quantum numbers. I guess at some point in the past two years, the itensor package has changed from using number to using string to initialize the SiteSet class. If I am correct, number 2 = "Up" and number 3 = "Dn".

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Thank you very much, hoi!
I have posted comments on github. Is it more convenient for you to anwer questions on github or here?