# SPINPACK

SPINPACK is a big program package to compute lowest eigenvalues and eigenstates and various expectation values (spin correlations etc) for quantum spin systems. These model systems can for example describe magnetic properties of insulators at very low temperatures (T=0) where the magnetic moments of the particles form entangled quantum states. The first SPINPACK version was based on Nishimori’s TITPACK (Lanczos method), but it was early converted to C/C++ and completely rewritten. Other algorithms are implemented too. See 2x2-diagonalization for example. It is able to handle Heisenberg, t-J, and Hubbard-Systems up to 64 sites if you have enough memory, disk space and computing power. For instance we were able to get the lowest eigenstates for the Heisenberg Hamiltonian on a 40 site square lattice on our machines. The package is written mainly in C to get it running on all unix systems. C++ is only needed for complex eigenvectors and twisted boundary conditions. The program can use all topological symmetries, S(z) symmetry and spin inversion to reduce matrix size. The results are very reliable because the package has been used since 1997 in scientific work.

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