1D SSH model DMRG code

Question

HI,

I am new to C language and iTensor.
I want to perform a DMRG calculation on the SSH model with Hamiltonian

$H=-\sum\i (1+\eta) c{A,i}^\dagger c{B,i} +(1-\eta) c{B,i}^\dagger c_{A,i+1} + h.c.$

I considered the A site as odd index site and B site as even index in a 1D chain. This is the code I used

#include "itensor/all.h"
using namespace itensor;
int main()
{
int N = 20;
Real eta = 0.5;

auto sites = Spinless(N);     
auto ampo = AutoMPO(sites);
for(int j = 1; j <= N; ++j)
    {
        if (j%2 == 1)
        {
            ampo += -(1+eta),"Cdag",j,"C",j+1;
            ampo += -(1+eta),"Cdag",j+1,"C",j;
        }
        else (j%2 ==0)
        {
            ampo += -(1-eta),"Cdag",j,"C",j+1;
            ampo += -(1-eta),"Cdag",j+1,"C",j;
        }
    }
auto H = MPO(ampo);
auto psi = MPS(sites);

auto sweeps = Sweeps(5);
sweeps.maxm() = 10,20,100,100,200;
sweeps.cutoff() = 1E-10;
sweeps.niter() = 2;
sweeps.noise() = 1E-7,1E-8,0.0;
println(sweeps);

auto energy = dmrg(psi,H,sweeps,"Quiet");

printfln("\nGround State Energy = %.10f",energy);

return 0;
}

However, the Hamiltonian seems haven't been created properly. The program terminated at a line saying "Using approx/svd conversion of AutoMPO->MPO"... May someone suggests what's wrong in my code?

Thanks in advance.

Answer 1

Hi, so if you get a 'hard' error like that (a crash) then probably something was out of bounds i.e. an integer value like j took on too big or too small a value. That's the most common reason for crashes in computer programs.

So looking at your code, the loop index "j" runs from 1 up to and including N. Then in your AutoMPO construction you are putting operators at j+1, which would be N+1, which is past the edge of your system.

Please try changing the loop stopping condition to j < N and see if you get sensible results after that.

Miles