prune=False flag in freeze_orbitals seems have no effect

[davidzsoltmanrique]

OpenFermion/src/openfermion/utils/_operator_utils.py

Line 165 in 02a0088

new_operator = prune_unused_indices(new_operator)

[ncrubin]

Hi @davidzsoltmanrique, thanks for flagging this. Could you provide a bit more information about this issue? maybe a code snippet you are using to test the effect?

[ncrubin]

Closing due to inactivity.

[mafaldaramoa]

Hello,

I came across this problem also.

Here's a snippet:

from openfermion import FermionOperator
from openfermion.transforms import freeze_orbitals

operator = FermionOperator("1^ 0",1) + FermionOperator("3^ 2",0.5)
print(f"Original Operator:\n{operator}")
new_operator = freeze_orbitals(operator,occupied=[0,1],prune=False)
print(f"\nNew operator:\n{new_operator}")

Output:

Original Operator:
1 [1^ 0] +
0.5 [3^ 2]

New operator:
0.5 [1^ 0]

It eliminated the 1 [1^ 0] term as expected, but renumbered the 2,3 orbitals as 0,1, even though I set prune=False. So now the 0.5 [3^ 2] term reads 0.5 [1^ 0], which is not what I wanted.

I took a look at the source code for freeze_orbitals and at no point is the prune flag used.