I use this matrix entry for further calculation, however by putting the “index” to 165, the calculation of the eigensystem takes a lot of time without result. I use Mathematica 12 in Windows and Linux.
you can see my test video here.
This issue first questioned here.
I tried Eigenvectors[H], and it gives an answer rather quickly. I don’t know if it is correct.
Thanks for your try.
I realize it is machine-dependent. I checked it on two computers and it encounters the mentioned problem. However, in one system it gives results fast. Also adding a small complex number (10^-20 i) to the index resolve the problem.