Fortran90 library of continuous-time loop algorithm

The Looper library is a library for quantum Monte Carlo (QMC) method, implementing the multi-cluster loop algorithm in the continuous-time path-integral representation. The library supports XXZ models with arbitrary spin size S on any bipartite lattices. Since it supports so-called freezing graphs, models with easy-axis (Ising-like) anisotropy can also be simulated very efficiently. It also supports parallelization using MPI.

Since 2001, development of a new version of the Looper was started, using C++ generic programing technique. The first official C++ version, which is now renamed as the ALPS/looper library and published as a part of the ALPS project, has been released June 2004. For more details about the ALPS/looper library, please visit the ALPS/looper web page.


1997 version 1 FORTRAN77 only works for S=1/2, bipartitle lattice, Heisenberg and easy plane anisotropy
1998/10/09 version 2.0 Fortran90, large S support
version 2.1.0
 -version 2.1.4
version 2.2.0
 -version 2.2.9
Parallelization (MPI) support
2000/10/31 version 2.3 Freezing (easy-axis anisotropy) support
2001/08/16 version 2.9 Experimental version in C++, non-bipartite lattice support, SSE representation support
2004/06/11 version 3 C++ version based on ALPS framework (ALPS/looper Library)


  • version 1: Synge Todo
  • version 2.0-2.2: Synge Todo and Kiyoshi Kato
  • version 2.3: Synge Todo, Kiyoshi Kato, and Shigeru Koikegami
  • version 2.9-: Synge Todo

Library Source Code

The source code is available freely from the author. Please contact Synge Todo.

Questions and request for support

The Looper library (version 2.9 or prior) is not supported anymore. Please use the ALPS/looper library instead.


  1. H.G. Evertz, The loop algorithm, Adv. in Physics, 52, 1 (2003).
  2. S. Todo and K. Kato, Cluster Algorithms for General-S Quantum Spin Systems, Phys. Rev. Lett., 87, 047203 (2001).
  3. S. Todo, Parallel Quantum Monte Carlo Simulation of S=3 Antiferromagnetic Heisenberg Chain, Computer Simulation Studies in Condensed-Matter Physics XV, ed D.P. Landau, S.P. Lewis, and H.-B. Schuettler (Springer-Verlag, Berlin, 2003) pp. 89-94.