thank you for your quick answer. Actually I have considered both periodic and open boundary conditions.
<SITEOPERATOR name="Sx" site="x">
1/2*(Splus(x)+Sminus(x))
</SITEOPERATOR>
<HAMILTONIAN name="Ising">
<PARAMETER name="J" default="0"/>
<PARAMETER name="h" default="0"/>
<BASIS ref="spin"/>
<SITETERM site="i">
J*h*2*Sz(i)
</SITETERM>
<BONDTERM source="i" target="j">
J*4*Sx(i)*Sx(j)
</BONDTERM></HAMILTONIAN>
And I run the following simulations: An exact
parms = [{
'LATTICE_LIBRARY' :"/MyIsing/lattice.xml",
'MODEL_LIBRARY' :"/MyIsing/model.xml",
'LATTICE' :"open chain lattice",
'MODEL' :"Ising",
'h' : 2.1,
'J' : -0.5,
'L' : 8
}]
input_file = pyalps.writeInputFiles('IsingEXACT',parms)
res = pyalps.runApplication('sparsediag',input_file,writexml=True)
and a DMRG
parms = [{
'LATTICE_LIBRARY' :"/MyIsing/lattice.xml",
'MODEL_LIBRARY' :"/MyIsing/model.xml",
'LATTICE' :"open chain lattice",
'MODEL' :"Ising",
'J' : -0.5,
'h' : 2.1,
'SWEEPS' : 4,
'NUMBER_EIGENVALUES' : 1,
'L' : 8,
'MAXSTATES' : 600
}]
The sparsediag gives me
-8.8210316277121077
and the DMRG gives me
-8.5812583500526003
with truncation error -2.2204460492503131e-16
Why the data are so distant notwithstanding the truncation error is so small, and it seems to be converged?
Kindest regards.