dmrg: a transverse-field site term (Sx) is silently dropped; the result is the field-free energy of a four-site chain
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Assessment
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Newbie friendliness
- 52/100
Research direction
The payload narrows the fault to dmrg, while sparsediag handles the same model term correctly; it does not name source files or tests. Start by tracing how dmrg receives and uses non-diagonal site terms when conserved quantum numbers are unset, then compare that path with sparsediag. Done means the Gamma cases return energies that depend on the field and agree with the reference values, or the input is explicitly rejected.
Written by the indexing model from the issue text.
Description
Summary
With any model containing the site term -Gamma*Sx(i), dmrg returns an
energy that does not depend on Gamma or on L and equals the ground-state
energy of a four-site open chain without the field. A diagonal site term
(-h*Sz(i)) is handled correctly. The program should either treat the term
or refuse the input.
Reproducer
LATTICE="open chain lattice"
MODEL="spin"
local_S=1/2
L=8
SWEEPS=4
MAXSTATES=60
NUMBER_EIGENVALUES=1
{Jz=1; Jxy=0; Gamma=0;}
{Jz=1; Jxy=0; Gamma=0.5;}
{Jz=1; Jxy=1; Gamma=0.5;}
{Jz=0.5; Jxy=1; Gamma=0.5;}
{Jz=1; Jxy=1; h=0.3;}
dmrg parm.in.xml; there is no conserved quantum number for the Gamma cases,
so CONSERVED_QUANTUMNUMBERS is left unset (setting it to an empty string
gives the same result).
Observed and expected
| parameters | dmrg E0 | exact E0 (dense diagonalisation) |
|---|---|---|
| Jz=1, Jxy=0, Gamma=0 | -1.750000 | -1.750000 |
| Jz=1, Jxy=0, Gamma=0.5 | -0.750000 | -2.459488 |
| Jz=1, Jxy=1, Gamma=0.5 | -1.616025 | -3.482240 |
| Jz=0.5, Jxy=1, Gamma=0.5 | -1.356086 | -2.992176 |
| Jz=1, Jxy=1, h=0.3 | -3.374933 | -3.374933 |
The three Gamma values are exactly the energies of the corresponding four-site
open chains without a field (-3/4 for three Ising bonds, -1.616025 for the
four-site Heisenberg chain). At L = 64 the result is the same -0.750000 for
Gamma = 0.3, 0.5 and 1.0. sparsediag on the same inputs agrees with the
exact values to 1e-14, so the model library builds the term correctly; the
loss is inside dmrg.
Observed on master at 0e2f2915d (Release build, GCC, OpenBLAS, HDF5 1.14, single-threaded), compared with independent reference calculations.
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- C++
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