[ALPS-users] mps_optim: measurement - bug?

[Michele Dolfi]

Dear Mateusz,

Which version of the code are you using?
We recently rewrote how measurements are parsed (specially for measurement expressions).

Could you please try with a version newer than r7807?


Best,
Michele

--
ETH Zurich
Dr. Michele Dolfi
Institute for Theoretical Physics
HIT G 32.4
Wolfgang-Pauli-Str. 27
8093 Zurich
Switzerland

dolfim at phys.ethz.ch
www.itp.phys.ethz.ch

+41 44 633 78 56 phone
+41 44 633 11 15 fax 

> On 14 Jan 2017, at 18:17, Mateusz Łącki <mateusz.lacki at gmail.com> wrote:
> 
> Dear All.
> I would like to report a likely bug.
> 
> I play with a simple transverse ising model for S=1/2. I  take a short chain of length say 8, compute magnetization in x and z direction. When I compute the same input with mps_optim and sparsediag i obtain different results. Both codes give me different magnetizatios in mps_optim measurement seems to be wrong by a factor.
> 
> I changed the definition of a default model  “spin” (see below), the coupling is along x axis and field is along z. I consider the following input:
> 
> LATTICE="inhomogeneous chain lattice"
> MODEL_LIBRARY="models.xml"
> 
> MODEL="spin"
> J=1
> SWEEPS=8
> chkp_each=8
> 
> MEASURE_LOCAL[Local magnetization Xa]=Splus
> MEASURE_LOCAL[Local magnetization Xb]=Sminus
> MEASURE_LOCAL[Local magnetization X1]=Sx
> MEASURE_LOCAL[Local magnetization X2]=Sxx
> MEASURE_LOCAL[Local magnetization Z]=Sz
> MAXSTATES=40;
> NUMBER_EIGENVALUES=1;
> 
> {h=0;Gamma=0;L=8} 
> 
> *******************
> In the above the Sxx operator is defined exactly the same as Sx:
> <SITEOPERATOR name="Sx" site="x">
>  1/2*(Splus(x)+Sminus(x))
> </SITEOPERATOR>
> 
> but with 1/4 factor, not 1/2:
> <SITEOPERATOR name="Sxx" site="x">
>  1/4*(Splus(x)+Sminus(x))
> </SITEOPERATOR>
> 
> 
> There are 2 runs that are important for my message:
> 
> RUN A):
> running the above input with sparsediag gives (at any site):
> Local magnetization Xa=0.5
> Local magnetization Xb=0.5
> Local magnetization X1=0.5
> Local magnetization X2=0.25
> 
> Which makes sense, as Sx=0.5*(Jplus + Jminus)
> 
> RUN B):
> running the above input with mps_optim gives (at any site):
> Local magnetization Xa=0.5
> Local magnetization Xb=0.5
> Local magnetization X1=1.0
> Local magnetization X2=1.0
> 
> 
> My conclusion:
> It seems that measurement ignores the factor 1/4 in the definition of Sxx and 1/2 in the definition of Sx. If it is indeed the case (not stupid mistake on my side), would it be possible to issue a patch?
> 
> If I change 
>  <BONDTERM source="i" target="j">
>    <PARAMETER name="J#" default="J"/>
>    -J#*Sx(i)*Sx(j)*4
>  </BONDTERM>
> 
> into 
>  <BONDTERM source="i" target="j">
>    <PARAMETER name="J#" default="J"/>
>    -J#*Sxx(i)*Sxx(j)*4
>  </BONDTERM>
> 
> then I get a correct factor 4 reduction in energy. So it seems only the mesurement ignores the numerical factor.
> 
> If I exchange x-z direction in the Hamiltonian everything seems fine.
> 
> Best,
> Mateusz Łącki
> 
> 
> 
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