Evaluate energy expectation value in infinite Purification MPS

[rmldj]

I ran into trouble trying to evaluate energy expectation value in an infinite purification MPS (which will model a thermal state). I installed the newest version of TeNPy (1.1.0) using conda today.

My setup is as follows:

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from tenpy.models.tf_ising import TFIChain
from tenpy.networks.purification_mps import PurificationMPS

model_params = {"L": 2,"J": 1.0, "g": 1.0, "bc_MPS": "infinite"}
model = TFIChain(model_params)
sites = model.lat.mps_sites()
psi = PurificationMPS.from_infiniteT(sites, bc="infinite", form="B")

Then in principle I run imaginary PurificationTEBD.

Then when I try

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print(psi.expectation_value(model.H_MPO))

I get AttributeError: 'MPO' object has no attribute 'rank'
while

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print(model.H_MPO.expectation_value(psi))

gives ValueError: different rank!

What is the correct way to evaluate such a thermal expectation value in the PurificationMPS formulation?
In the documentation I could just find examples for spin operators.

I would be grateful for any help, as I am a complete novice in TeNPy...
Romuald

[Jakob]

Hi!

In MPS.expectation_value, use the axes parameter.
Hope this already helps, if not, post again =)

Cheers
Jakob