WriteM argument for idmrg

Question

Hi,

I am trying to use the WriteM of the idmrg but it shows an error message.

For example, I used the idmrg.cc file in the sample folder and add an input arg {"WriteM",10} such that the environment tensor of the dmrg can be written on disk when MaxM>10.

However, it prints the follow error message

Wrong number of IndexVals passed to real/cplx (expected 1, got 0)
Abort trap: 6

It seems like the problem is in the localmpo.h file. In the setRHlim and setLHlim function, I saw

if(LHlim_ < 1) 
    {
    //Set to null tensor and return
    PH_.at(LHlim_) = Tensor();
    return;
    }

and

 if(RHlim_ > Op_->N()) 
    {
    //Set to null tensor and return
    PH_.at(RHlim_) = Tensor();
    return;
    }

respectively.

These two line works for finite size DMRG because in the finite size dmrg, there is no PH[0] and PH[N+1], where N = Op_->N(). You can set it as a empty tensor. But for the idmrg, there are edge tensor to terminate the finite state machine. If we set it to be empty tensor, we will get the error.

I think one can add a private member like

bool Infinite_ = arg.getbool("Infinite",false).

The "Infinite" argument should have been defined in the idmrg.cc file.
And change the previous line to something like

if(RHlim_ > Op_->N() && !Infinite_) 
    {
    //Set to null tensor and return
    PH_.at(RHlim_) = Tensor();
    return;
    }

to avoid the problem.

Answer 1

Hi Ze-Pai,
Basically I think the answer to your question is that WriteM is currently not intended to be used with the idmrg algorithm. This can certainly be fixed, but I would currently view this as not a supported feature. We should add it to the list of new features to add to ITensor.

One question back to you though: are you sure that you need idmrg for your project? Perhaps you could use finite DMRG just as well?

Finally, thanks a lot for looking into the code to diagnose the problem. Maybe it is a simple fix after all like you are suggesting. Do you want to test your fix out on your own code branch then submit a pull request if it does work?

Miles