Dear all,
I am trying to compute the correlation of two bond operators as follows:
p['MEASURE_CORRELATIONS[custom op]'] = 'cppdag:cpp'
where cpp and cppdag are define in mymodel.xml as:
<BONDOPERATOR name="cpp" source="x" target="y">
c_up(x)*bdag(x)*c_up(y)*bdag(y)
</BONDOPERATOR>
<BONDOPERATOR name="cppdag" source="x" target="y">
b(x)*cdag_up(x)*b(y)*cdag_up(y)
</BONDOPERATOR>
where bdag, b, cdag_up and c_up are the usual boson, fermion creation-annihilation operators.
Disappointingly, in this case mps_optim only outputs a single number for the correlation measurement.
If I try instead to compute the correlation 'oppdag:opp' with
<BONDOPERATOR name="opp" source="x" target="y">
bdag(x)*c_up(y)
</BONDOPERATOR>
<BONDOPERATOR name="oppdag" source="x" target="y">
cdag_up(x)*b(y)
</BONDOPERATOR>
... I get a 4x4 matrix (i,i+1,j,j+1) as expected.
So the problem seems to come from computing the correlation of bond operators that are products of more than two site operators.
Is this measurement expected to fail with mps_optim, or am I doing something wrong?
Thank you for your help.
Michael