ground energy_square lattice

Hello! I would like to calculate the ground state of free spinless fermions on a square lattice 4x4. Hamiltonian is the same as in [(http://weinbe58.github.io/QuSpin/generated/quspin.basis.spinless_fermion_basis_general.html#quspin.basis.spinless_fermion_basis_general)] but with mu=0 and U=0.

1. When I write basis_2d=spinless_fermion_basis_general(N_2d, Nf=1, kxblock=(T_x,0),kyblock=(T_y,0),pxblock=(P_x,0),pyblock=(P_y,0))? I got the warning

 GeneralBasisWarning: using non-commuting symmetries can lead to unwanted behaviour of general basis, make sure that quantum numbers are invariant under non-commuting symmetries!
  exec(compile(f.read(), filename, 'exec'), namespace)

1. when I set the number if fermions Nf=2 I got the error:

ValueError: zero-size array to reduction operation minimum which has no identity

1. I also tried this one basis_2d=spinless_fermion_basis_general(N_2d,Nf=2)

but the ground energy (E_g) seems to be wrong: I expect (the hopping J=1) E_g =-4 for Nf =1 E_g =-6 for Nf =2 E_g =-8 for Nf =3 E_g =-8 for Nf =4 E_g =-8 for Nf =5 and so on but I got E_g =-10 for Nf =4

PS. Is it possible to delete my previous issue?

[mgbukov]

We were able to reproduce the bug. We’ll try to fix it asap.

1): the warning message tells you that the momentum and parity symmetries do not commute. In general, this is true. However, in the momentum k=0,pi sectors they do commute. The warning is to make the user alert, so they can double check by turning on and off the symmetries.

In principle, if one ignores the warning, the results could be wrong.

“Should I insert kxblock=(T_x,0),kyblock=(T_y,0),pxblock=(P_x,0),pyblock=(P_y,0) for simple square lattice with periodic boundary conditions?”

This depends on your Hamiltonian.

A good review which explains the symmetries is https://arxiv.org/abs/1101.3281

We’ll let you know once the issue has been fixed.

[mgbukov]

Seems like I was wrong, too. The spinless_fermion_general was fine all along, there is indeed no bug.