On 05 Sep 2016, at 06:44, Natanael de Carvalho Costa <natanael@if.ufrj.br> wrote:

Dear All,

I'm using ALPS DMRG (MPS) to study the magnetism of XY model in a frustrated chain.

In order to obtain the Sx correlation functions, I add the following command line: MEASURE_CORRELATIONS[Sx_correlation]= Sx

I expected that the output file presented me all <Sx(i) Sx(j)> correlation functions. However, there were only this line in the output file. 

<SCALAR_AVERAGE name="Sx_correlation"><MEAN>6.7066684180584175e-317</MEAN></SCALAR_AVERAGE>

Even when I chose to program make only specifics calculations, such as 

MEASURE_LOCAL_AT[SxSx_corr] = "Sx:Sx|(0,1),(0,2),(0,3),(0,4)"

the answer I obtained was zero, no matter of the choice of the external parameters, as showed below.

<VECTOR_AVERAGE name="SxSx_corr">
<SCALAR_AVERAGE indexvalue="( 0 ) -- ( 0 )"><MEAN>0</MEAN></SCALAR_AVERAGE>
<SCALAR_AVERAGE indexvalue="( 0 ) -- ( 0 )"><MEAN>0</MEAN></SCALAR_AVERAGE>
<SCALAR_AVERAGE indexvalue="( 0 ) -- ( 1 )"><MEAN>0</MEAN></SCALAR_AVERAGE>
<SCALAR_AVERAGE indexvalue="( 0 ) -- ( 1 )"><MEAN>0</MEAN></SCALAR_AVERAGE>
</VECTOR_AVERAGE>

*(the index are not the same in the input and output file because I'm working with a unit cell with 3 sites.)

Owing to these not good results, I did a test, and then I measured the correlation functions <Splus(i) Splus(j)>, <Splus(i) Sminus(j)> and <Sminus(i) Sminus(j)>, as showed below

MEASURE_LOCAL_AT[SxSx_corr1] = "Splus:Splus|(3,18)"
MEASURE_LOCAL_AT[SxSx_corr2] = "Splus:Sminus|(3,18)"
MEASURE_LOCAL_AT[SxSx_corr3] = "Sminus:Splus|(3,18)"
MEASURE_LOCAL_AT[SxSx_corr4] = "Sminus:Sminus|(3,18)"

and I obtained 

 <VECTOR_AVERAGE name="SxSx_corr1">
 <SCALAR_AVERAGE indexvalue="( 1 ) -- ( 6 )"><MEAN>-0.1565028679350004</MEAN></SCALAR_AVERAGE>
 

</VECTOR_AVERAGE>
<VECTOR_AVERAGE name="SxSx_corr2">
<SCALAR_AVERAGE indexvalue="( 1 ) -- ( 6 )"><MEAN>-0.15912792472820933</MEAN></SCALAR_AVERAGE>

</VECTOR_AVERAGE>

<VECTOR_AVERAGE name="SxSx_corr3">
<SCALAR_AVERAGE indexvalue="( 1 ) -- ( 6 )"><MEAN>-0.15912792472820814</MEAN></SCALAR_AVERAGE>
</VECTOR_AVERAGE>

<VECTOR_AVERAGE name="SxSx_corr4">
<SCALAR_AVERAGE indexvalue="( 1 ) -- ( 6 )"><MEAN>-0.15650286793499901</MEAN></SCALAR_AVERAGE>
</VECTOR_AVERAGE>

which means that <SxSx>'s are really nonzero. Actually, these last results are in good agreement with exact diagonalization for a small chain. (Before you asked me, I correctly defined the onsite operator Sx in the model library.)

Then, probably it could be a bug when the program is summing the terms of SxSx correlations.

I really thank you for this great program.

Regards,

Natanael C. Costa

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