TeNPy Forum

Hi,
I want to calculate entanglement entropy in a finite two-leg ladder model, where the cutting bonds are two horizontal bonds in two legs. As an example, if I consider 6*2 lattice, and it is written in [0,1,2,3,.....11] in a zig-zag fashion, the bonds I want to cut are ( between 4 and 6) and (between 5 and 7). Cutting these two bonds will divide the whole ladder into two half: left and right. This can be achieved by putting the whole segments at the left and by using entanglement_entropy_segment, which is very costly for a large segment. As I am considering just two bonds (simultaneously), is there another way to do the calculation? ( I understand that if I just want to cut one bond, how easy it is being part of DMRG, but I want two bonds to be cut). It seems to me that mentioning [bonds] works for a single cut.

Let's look at the example Ladder image:


The MPS follows the red dashed line 0 - 1 - 2 -3 -4 -5- ...
When you cut the MPS at the bond between 3 and 4, it will split the system in regions left = {0, 1, 2, 3} and right={4, 5, 6, ...}.
This is exactly what you want!
Be careful about the indexing, though!
entanglement_entropy takes an argument bonds.
bonds=[i] will cut between sites (i-1, i), left of site i, so you should use e.g. bonds=range(2, L, 2)