Heisenberg cylinder infinite Hamiltonian

ITensor Julia Questions
1

Hi!

I found an example of the four-leg Heisenberg cylinder in the InfiniteMPS package. In that example, the Hamiltonian is defined as below.

function ITensorInfiniteMPS.unit_cell_terms(::Model"heisenberg2D"; width)
  opsum = OpSum()
  for i in 1:width
    # Vertical
    opsum += -0.5, "S+", i, "S-", mod(i + 1, width)
    opsum += -0.5, "S-", i, "S+", mod(i + 1, width)
    opsum += "Sz", i, "Sz", mod(i + 1, width)
    # Horizontal
    opsum += -0.5, "S+", i, "S-", i + width
    opsum += -0.5, "S-", i, "S+", i + width
    opsum += "Sz", i, "Sz", i + width
  end
  return opsum
end

The mod function links the third site and the zeroth site. Similarly, there is a link between the first site and the fourth site. I wonder if one should use mod1 instead of mod here.