Hello!

I want to compute the following matrix elements:

The Hamiltonian (H) is already stored as an MPO and Phi_0 is also stored as an MPS. Below is the code I am trying right now:

```
for (int i=0; i<N; i++)
{
for (int j=0; j<N; j++)
{
auto ampo = AutoMPO(sites);
ampo += "Cdagup", i, "Cup", j;
//ampo += "Cdagdn", i, "Cdn", j;
MPO op = toMPO(ampo);
auto C = nmultMPO(H[point],op,{"MaxDim",500,"Cutoff",1E-8}); cout << "os edo kala\n";
auto D = nmultMPO(op, H[point], {"MaxDim",500,"Cutoff",1E-8}); cout << "os edo kala\n";
gsl_matrix_set(G, i,j,-inner(states.at(ind),C,states.at(ind))+inner(states.at(ind),D,states.at(ind)));
}
}
```

I am getting the following error: Unassigned site in SiteStore

Any idea about why? Is it because I only have two indices in the C,D MPO's?

What I am trying to do is to basically implement orbital optimization for the Hubbard Hamiltonian. I do not know if there is another way to do that in iTensor.