Documentationl:SSE
ALPS application: dirloop_sse</h1> The dirloop_sse package is one of the applications of the <a
href="http://alps.compphys.org">ALPS project</a>. It provides a full
generic implementation of the Quantum Monte Carlo (QMC) method called directed loop algorithm in the Stochastic Series Expansion representation. The dirloop_SSE method was invented and developped by <a
href="http://physics.bu.edu/~sandvik">Anders Sandvik </a>and
coworkers. It is a powerful and elegant QMC method to study quantum
spin or bosonic lattice models.
The current implementation we present here uses the most
recent developments of this method. This version allows to simulate on arbitrary
lattices :
 Quantum spin (even frustrated  see remark below) models with arbitrary spin size, magnetic field and anisotropy
 (Softcore) bosonic models
This release allows to simulate systems with a sign problem (e.g. frustrated spin systems).
However, this case was moderately tested so please be careful if your model has a sign problem ...
Please note that for frustrated models, some values of the parameter Epsilon might render the algorithm non ergodic (for example, when you have a "pure loop" algorithm). One needs to check this carefully.
Contents
Running a simulation
is discussed in the <a href="../../../doc/tutorial2.html">tutorial</a>.
Input parameters
In addition to the general input parameters of the <a
href="http://alps.compphys.org/software/alps/doc/scheduler/running.html">ALPS
scheduler library</a> the sse application takes the following input parameters:
<thead> </thead> <tbody> </tbody>Name  Default  Description 
LATTICE_LIBRARY  lattices.xml  path to a file containing lattice descriptions 
LATTICE  name of the lattice  
MODEL_LIBRARY  models.xml  path to a file containing model descriptions 
MODEL  name of the model (for example "spin" or "boson")  
T  the temperature  
BETA 
inverse of temperature (if
temperature is not given) 

SWEEPS 
number of Monte Carlo steps
(after thermalization) 

THERMALIZATION  SWEEPS / 10  Number of Monte Carlo sweeps for thermalization 
In addition, the lattice description can require further parameters
(e.g. L or W) as specified in the lattice
description file.
The model description can also require further parameters (e.g. S=1/2
or S=1, h=0.5 for spin models, t=1.5 or mu=0.5
for boson models) as specified in the model description file.
Parameters for experts
In addition, specific simulations parameters can be assigned (use only if you see what it means !):
<thead> </thead> <tbody> </tbody>Name  Default  Description 
SKIP  1  the number of Monte Carlo sweeps between each measurement 
NUMBER_OF_WORMS_PER_SWEEP  Calculated self consistently  number of worms done during the
loop update. By default, this number is calculated selfconsistently during the thermalization part. Nevertheless, you can force its value during the whole simulation 
EPSILON  0  supplementary diagonal energy shift for all interactions. The
value of Epsilon affects the performances of the algorithm with the following tradeoff : the higher it is, the longer the simulation time but the lowest are bounce probabilities. Current wisdom indicates that one should use nonzero values for Epsilon, but not too high (for example S/2 for spin S models). Please note that for frustrated models, some values of Epsilon might render the algorithm nonergodic. You have to check carefully. 
WHICH_LOOP_TYPE  "minbounce"  string to indicate which type of updates should be used for the scattering at the vertices : ( "heatbath" ) heatbath, see A. W. Sandvik, Phys. Rev. B 59, 14157 (1999). ( "minbounce" ) minimum bounces, see F. Alet, S. Wessel, and M. Troyer Phys. Rev. E 71, 036706 (2005). ( "locopt" ) locally optimal, see L. Pollet, S. M. A. Rombouts, K. Van Houcke, and K. Heyde, Phys. Rev. E 70, 056705 (2005). By default the algorithm uses the "minbounce" updates. 
NO_WORMWEIGHT  0  boolean to indicate whether the worm matrixelement should be set to unity (NO_WORMWEIGHT = true) or to its real value depending on the spin/density configuration (NO_WORMWEIGHT=false). By default, NO_WORMWEIGHT is false. 
Measurements
The following observables are measured by the sse application:
<thead> </thead> <tbody> </tbody>Name  Description 
Energy  total energy of the system 
Energy Density 
energy per site 
Magnetization  the zcomponent of the magnetization (spin models) 
Magnetization  absolute value of the zcomponent of the magnetization (spin models) 
Magnetization^2  square of the zcomponent of the magnetization (spin models) 
Staggered Magnetization  the zcomponent of the staggered magnetization (bipartite lattice and spin models) 
Staggered Magnetization^2  square of the zcomponent of the staggered magnetization (bipartite lattice and spin models) 
Susceptibility  the uniform susceptibility (spin models) 
Density  particle number (for bosonic models) 
Density^2  square of the particle number (for bosonic models) 
Stiffness 
stiffness of the system (both
for spin and bosonic models) 
Other observables might also be available depending on the exact
version of the sse
application.
License
The license allows the use of the applications for noncommercial scientific use provided that the use of the ALPS applications and libraries is acknowledged and referenced in any scientific publication, as discussed in this <a href="http://alps.compphys.org/software/applications/LICENSE.txt">license file</a>. Even though this is not mandatory, we would be happy to hear from you (for example by email) if you used sse to produce data used in a scientific publication.
Questions and request for support
can be addressed to <a href="mailto:alet@compphys.org">Fabien Alet</a> and <a href="mailto:troyer@compphys.org">Matthias Troyer</a>.
Contributions and feature requests
We appreciate hearing your requests for additional features and also welcome any contributions to the ALPS project.
Contributors
The following persons have contributed to the sse application:
 <a href="http://www.itp.phys.ethz.ch/staff/alet/">Fabien Alet </a>(<a href="mailto:alet@compphys.org">alet@compphys.org</a>)
 <a href="http://www.itp.phys.ethz.ch/staff/troyer/">Matthias Troyer</a> (<a href="mailto:troyer@compphys.org">troyer@compphys.org</a>)
<address>copyright (c) 20022004 by <a href="mailto:alet@compphys.org">Fabien Alet</a> and <a href="mailto:troyer@compphys.org">Matthias Troyer</a></address>