Fast exact diagonalization of translation-invariant spin chain

A Fortran program for fast exact diagonalization of translation-invariant spin chains.

Users only need to modify three user_*.f90 files to use this program for their own purposes.

TODO:

much more documentations coming soon

Remarks:

• Relative residuals as written in residuals-k.txt should be ignored (unless eigenvectors are computed).
• As spin chain gets larger, array sizee may exceed 2^31-1, and ILP64 interface layer must be used. Use makefile-i8 in that case.