Dear all, I would like to use MPS to calculate the correlation of four operators .The correlation of four operators like this cdag_up(i):cdag_down(i+1):c_up(i+1):c_down(i+2) I use the following two ways to calculate,but the results of the two methods are opposite. The first method: In model file <SITEOPERATOR name="cdag_down__c_up" site="x"> cdag_down(x)*c_up(x) </SITEOPERATOR> In command file MEASURE_LOCAL_AT[cdag_up:cdag_down:c_up:c_down] = "cdag_up:cdag_down__c_up:c_down|(0,1,2)" another method: In model file <OPERATOR name="ca" matrixelement="1"> <CHANGE quantumnumber="Ndown" change="1"/> <CHANGE quantumnumber="Nup" change="-1"/> </OPERATOR> In command file MEASURE_LOCAL_AT[cdag_up:cdag_down:c_up:c_down] = "cdag_up:ca:c_down|(0,1,2)" I would like to know which method is correct and why the answers are opposite? It is highly grateful for your any suggestions. Yours sincerely, Xingbo Wei Zhejiang Normal Universtiy, Jinhua, 230014, China