Implementing AutoMPO for thermal state calculations

Question

This may be a basic question, but I am trying to implement a thermal state calculation, and I am definitely doing something incorrectly. I think my problem lies in using AutoMPO.

At one point, I am calculating the operator S^zk (the Fourier transform of S^zi), and I build S^z_k using AutoMPO as

        auto ampo = AutoMPO(sites);
        for(auto s : lattice->getSites())
        {
            if(s.t == Lattice::physical)
            {
               ampo += sin(0.5*(s.s+1)*q),"Sz",s.s;
            }
        }

sites is of type SpinHalf with 2N sites, s.t is a tag as either "physical" or "environment", and s.s is the site number. all physical sites are odd, and ancilla sites are even.

I also need to calculate the Louivillian defined by L = HxI - IxH, with H the Heisenberg Hamiltonian. My code for this is

    auto ampo = AutoMPO(sites);
    for(auto b : lattice->getBonds())
    {
        if(b.t == Lattice::physical)
        {
            ampo += 0.5,"S+",b.s1,"S-",b.s2;
            ampo += 0.5,"S-",b.s1,"S+",b.s2;
            ampo +=     "Sz",b.s1,"Sz",b.s2;
        }
        if(b.t == Lattice::environment)
        {
            ampo += -0.5,"S+",b.s1,"S-",b.s2;
            ampo += -0.5,"S-",b.s1,"S+",b.s2;
            ampo += -1.0,"Sz",b.s1,"Sz",b.s2;
        }
    }
    L = toMPO(ampo);

My main question is, can I recycle AutoMPO for use with a thermal state or do I need to modify something else to do this? And how do I correctly implement tensoring with the identity on the ancilla degrees of freedom?

Thank you for your time,
Nick

Answer 1

So I figured it out, it was somewhere else in my code that the issue was arising. For anyone else having the same issue, AutoMPO automatically puts the identity on operators that are not specified.

Comments

Thanks for the update!