[ALPS-users] Defining pair correlation for fermions?

Hi, I'm a new user of ALPS and I have some trouble with defining pair correlations. The problem I'm studying is spin-1/2 fermions in one-dimension. I want to measure the pair correlation, namely the correlation function of c_up(i) c_down(i). I learnt from the ALPS tutorial that to measure the correlation of certain operator I have to define it in "sitebasis" first. Naively I can define the pair operator as

<OPERATOR name="pair" matrixelement="1"> <CHANGE quantumnumber="Nup" change="-1"/> <CHANGE quantumnumber="Ndown" change="-1"/> </OPERATOR>

and I add this to the definition of sitebasis of "fermion". It seems the only way to implement this operator, but what confuses me is the sign: how do this definition know that c_up and c_down are anti-commuting? It seems to me that the matrixelement can be 1 or -1, depending on how the basis state is defined. So anyone can tell me whether this is the right way to go?

Thanks a lot.

with all best,

Meng Cheng Graduate Research Assistant Department of Physics University of Maryland College Park http://terpconnect.umd.edu/~chmeng