

In [1]:

    
import quimb as qu

H = qu.ham_heis(12)



In [2]:

    
%%timeit
el, ev = qu.eigh(H)









    



8.81 s ± 566 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

This is already quite slow, and will not be sustainable for larger lengths. The Heisenberg Hamiltonian however has an abelian $Z_2$ subgroup symmetry, conserved Z-spin, that manifests itself in the computational basis. Thus if instead we specify autoblock=True:



In [3]:

    
%%timeit
el, ev = qu.eigh(H, autoblock=True)









    



372 ms ± 192 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)



In [4]:

    
psi = qu.eigvecsh(H, k=1, which='sa')