Unexpected bond dimension

[Shovan-Physics]

I find that mps.info.get_max_bond_dimension() is sometimes larger than the maximum bond dimension allowed during sweeps.

I am looking at the SU(2) t-J model on a ladder, using the example in https://block2.readthedocs.io/en/latest/tutorial/custom-hamiltonians.html#SU(2)-t-J-Model and the following parameters:

LX, LY = 20, 2
L = LX * LY
J = 0.0
N_ELEC = L-1
TWO_S = L-1

bond_dims = [100] * 4 + [200] * 4

I get a ground-state energy E = -2.97766 and mps.info.get_max_bond_dimension() = 441, even though the maximum allowed bond dimension was 200.

If I use the modified symmetry type suggested here: #115 (comment) (required for converting to SZ), I get an even higher bond dimension for the same parameters, with mps.info.get_max_bond_dimension() = 860.

The mismatch persists for nonzero J and lower values of TWO_S.

How do I understand this? Is there a way to track the actual (maximum) bond dimension at the end of each sweep?

[hczhai]

Thanks for reporting this problem. The value returned from mps.info.get_max_bond_dimension() is incorrect.

As a workaround, you can add an argument full_fci=False to driver.get_random_mps, then mps.info.get_max_bond_dimension() will behave correctly.

This will be fixed in version 0.5.3rc19 so that mps.info.get_max_bond_dimension() will print the correct value with or without full_fci=False.

[Shovan-Physics]

Thanks for the quick response.

As a related question, does block2 have any ''observer" feature that can be used to track the actual bond dimension or some expectation value during the sweeps?

[hczhai]

After you upgrade block2 to version 0.5.3rc19 (with pip options --upgrade --force-reinstall --no-deps --no-cache-dir), you can add the following before driver.dmrg:

mps = ...
mpo = ...

@driver.set_callback
def observer(event, iprint):
    if event == "DMRG::sweep.end" and iprint > 0:
        print('Sweep Mmps =', mps.info.get_max_bond_dimension(), 'Expt =', driver.expectation(mps, mpo, mps))

driver.dmrg(...)

Then at the end of each sweep, the observer function will be executed.

[Shovan-Physics]

Excellent, thanks.