Hi,

I'm still struggling with defining my own site class for a Hubbard-like model for two orbitals per site. I noticed the following behaviour, which is kind of weird for me. To debug my program, I calculated all the matrix elements of two-site Hamiltonian and I found that I don't understand what is going on with signs when a creation-annihilation operator pair was applied. To further clarify it I have artificially set all the elements of F operator to +1. And I arrived at following situation:

- I have a term in autoMPO:

ampo += t,"CdagAUp",j,"CAUp",j+1;

ampo += t,"CdagAUp",j+1,"CAUp",j;

- t=0.5

- All the elements of CAUp and CdagAUp are +1 or 0

- All the elements of F are +1 or 0

- And yet, the matrix element of a two site Hamiltonian between states connected by these elements is -0.5.

So, the question is: am I missing something about the fermionic operators? I know that in the Hubbard class there are Fup and Fdn operators - should I write analogous operators for myself? What are they for? (in the tutorial on Jordan-Wigner strings there is a F_j operator for spinful Jordan-Wigner transformation - are they somehow related to it? I assumed that all the sign changes related to swapping operators within one site are included in C, Cdag operators, but maybe I'm wrong.)