Structural Bioinformatics Library
Template C++ / Python API for developping structural bioinformatics applications.

Authors: A. Chevallier and F. Cazals
The WangLandau (WL) algorithm ([170], [111] [84]) is a recently develop stochastic adaptive algorithm targeting two primary goals
Recent improvements target in particular the random walk used by WangLandau. This package presents these improvements [52].
Intuitively, the density of states (DOS) of a physical system refers to the number of states (volume in phase space) of the system available at each energy level (whence the term density), which is especially useful to compute partition functions in statistical physics, and more generally observables–e.g. the average energy or the heat capacity.
More formally, we consider a measure defined up to a constant – finite measure rather a probability measure). In practice, we consider a distribution with density defined on a subset .
Also consider a partition of into socalled strata . Denoting the Lebesgue measure, our problem is to estimate
This problem arises in many areas of science and engineering, two of them being of particular interest in the sequel.
At time , the WL algorithm computes a sequence of estimates .
Using WL, a multidimensional integral gets approximated by a discrete sum of function values multiplies by the measure of points achieving a given value [18].
In practice, the method proposed by [13] to compute an integral relies on the following observation:
Following the points distribution in each stratum given by Eq. (eqwlbiaseddensity), the average value in a bin is defined by the integral
This integral can be computed during WL runtime by computing the average energy value in a given bin. Therefore if the total volume is known, the integral can be computed with WL.
The WL algorithm iteratively construct a sequence of estimates for the unknown vector .
Let be the index of the stratum containing point Points sampled in a given stratum by WL at time follows the probability density
To do so, the main steps are (see pseudocode):
We note that the exponential regime is governed by a flat histogram, which allows checking that all bins are visited evenly. Denoting be the number of samples up to iteration falling into the ith stratum . The vector is said to verify the flat histogram (FH) criterion provided that, given a constant :
The WangLandau algorithm 
The previous algorithm uses a proposal to propose a novel sample/conformation from the current one .
This proposal is key to ensure that WL will diffuse across all strata.
Following [52], we resort to the notion of diffusivity across strata, using socalled aggregated transition matrices (ATM) and climbing times [52].
Samples accepted are also used to measure the time taken to travel across all strata:
This said, running WL requires the following:
Physical system and proposal/move set.
Main output. The algorithm saves datas contained in the WL Data Structure a given times on a class called T_DS_snapshot. This data structure contains by default:
These pieces of information can be either saved at fixed intervals or at intervals that scales exponentially with time for log plots.
For each case
The software package is divided into 5 major components:
Physical system.. The physical system , provided by the user, specifies the state space and the energy function. It may also provide the gradient of the energy function, if used in conjunction with move set requiring differential information.
Move set and its controller.. The move set/proposal provides the mechanism to generate a novel conformation given an existing one, and also provides transition probabilities used to ensure detailed balance. The move set works in close collaboration with its socalled controller – see below.
WL algorithm. The Wang Landau class SBL::GT::T_Wang_Landau is the core one, which glues all pieces together and delivers the DoS estimation. It relies in particular on the WL data structure, which stores bin ranges, additional pieces of information detailed below, and provides the bins management policy. To perform specific processing, like numerical integration, the user must defined a custom WL DS, by inheritance from the default (provided) one.
Helper classes for simulations.. We also provide the class SBL::GT::T_WL_Simulation , which encapsulates the main ingredients and eases the task of launching a simulation on a given physical system using a given proposal.
To present the data exchange between the main components (Fig. figdataflow ), we introduce the following parameters and statistics:
: move set parameters (Class Move_Params ).
: statistics gathered during the generation of a novel conformation by the move set, and used to tune adaptivity (Class Move_Stats ).
: statistics, on a per bin basis, aggregating the observed values of along the simulation (Class Controller_Data ).
Data flow and update of data structures upon generation of a point by the move set. Arrows indicate the ordering for data exchange between the main components. See [52] for details. 
A Physical system must define the following components:
The Conformation class must define:
The main classses are
A Move set Traits class must define two classes templated by Conformation:
The Move_Set_Class must:
The Controller class must:
The following move sets classes are provided:
The SBL::GT::T_WL_Simulation< Physical_System , Move_Set_Traits > class
Virtual functions of T_WL_Simulation:
In addition, the library propose partial specialisation of T_WL_Simulation for the default move sets provided by the SBL that defines the set_ms_controller_params and set_move_set_params functions,
The functor SBL::GT::WL_histo_change_measure provides the mechanism to use WL by changing the measure.
We create the Physical system with:
example/isotrop_energy_well_physical_system.hpp:
Then use the WL_Simulation helper class. First we specialize the Simulation class for our Physical System. This specialisation main role is to create initial bins of size 0.1 for energies in [0,1], and to give the dimension to the physical system.
example/isotrop_energy_well_figures.cpp:
The Ising model is discrete therefore a custom move set has to be defined. For this example, the WL_Simulation class will not be used since it was designed around the predefined continuous move sets. Besides, it allows us to see how to set up Wang_Landau for start to finish.
The ising model for a 2D grid:
Ising Physical System:
Then we define a Move Set that only do flips. Since this is a symetric move ( i.e. the probability of going from A to B is the same than going from B to A), we can define the probability of going from A to B as 1 (the normalisation does not matter here, this term will be simplified in the Metrpolis criterion anyway). There is no adaptivity here so the Move Set controller is doing nothing.
Finally, here is the overall file defining the Physical System and Move Set in a simulation class/ launching the simulation File: example/Wang_Landau/Ising_figures.cpp
Finally we can define our WangLandau classes for L = 16:
using Move_Set_Traits = Spin_Move_Set_Traits; using Physical_System = Spin_Physical_System<16>; using WL_Ising = T_Wang_Landau<Physical_System,Move_Set_Traits>;
and launch the simulation:
struct Parameters{ uint64_t num_steps; double flat_hist; std::string output_prefix; std::string get_filename(){ return "Ising_flat_hist_" + std::to_string(flat_hist) + ".txt"; } }; void run(uint64_t num_steps,Parameters params){ WL_Ising::Parameters params_test_adaptative; WL_Ising::Physical_systems_parameters params_phy; WL_Ising::WL_DS_Parameters params_ds; WL_Ising::MS_controllers_parameters ms_controller_params; //nothing to set here WL_Ising::Move_Set_Parameters ms_params; //nothing to set here either //bin management: params_ds.base_ds_params.accept_new_bins = true; params_ds.base_ds_params.energy_levels_continuous = false; params_ds.base_ds_params.new_bins_size = 0.1; //flat histogram: params_ds.base_ds_params.histo_check_frequency = 100; params_ds.base_ds_params.flat_histogram_threshold = params.flat_hist; //here we show other parameters that can be set, but that can usually be left as default params_ds.base_ds_params.gamma_0 = 0.1; params_ds.base_ds_params.learning_rate = 1; params_ds.base_ds_params.density_updater_type = 2; //save parameters params_test_adaptative.save_type = DoS_save_frequency_type::Log_scale; params_ds.base_ds_params.save_events = false; params_test_adaptative.save_rate = 1.01; params_test_adaptative.debug_display_rate = 1000000; params_test_adaptative.roll_back_check_frequency = 2000; WL_Ising WL_instance(params_test_adaptative,params_ds,params_phy,ms_controller_params,ms_params); WL_instance.run_WL(num_steps); //save results std::string filename = params.get_filename(); //last snapshot: auto last_snap = WL_instance.get_snapshots()[WL_instance.get_snapshots().size()1]; //utility class to serialze results WL_stats_serializer serializer; //save the final histogram std::string histogram_filename = params.output_prefix+"histogram_" + filename; serializer.save_histogram(histogram_filename,last_snap); //save the connetivity graph std::string connectivity_filename = params.output_prefix+"connectivity_" + filename; serializer.save_connectivity(connectivity_filename,last_snap); //save climbs and descents std::string climbs_filename = params.output_prefix+"climbs_" + filename; std::string descents_filename = params.output_prefix+"descents_" + filename; serializer.save_climbs(climbs_filename,last_snap); serializer.save_climbs(descents_filename,last_snap); } int main(int argc, char* argv[]){ namespace po = boost::program_options; Parameters params; po::options_description desc("Allowed options"); desc.add_options() ("help,h", "produce help message") ("num_steps,n", po::value<uint64_t>(¶ms.num_steps)>default_value(1000000), "number of steps") ("flat_hist", po::value<double>(¶ms.flat_hist)>default_value(0.1), "flat histogram threshold") ("output_prefix", po::value<std::string>(¶ms.output_prefix)>default_value(""), "prefix to put in front of output files") ; po::variables_map vm; po::store(po::parse_command_line(argc, argv, desc), vm); po::notify(vm); if (vm.count("help")) { std::cout << desc << "\n"; return 1; } run(params.num_steps,params); }
See the following jupyter demos:
The function considered is U(x) = x^2, in dimension dim (=20 e.g.)
The following should be noticed:
Compilation of the executable, see paper:
Update of one's PATH: add ${SBL_DIR_INSTALL}/bin
Replication of results directly from python:
from SBL import SBL_pytools
from SBL_pytools import SBL_pytools as sblpyt
from SBL import WL_Boltzmann_single_well_example as wlbe
dim = 20
Ts = [0.05,100000,0.01,0.1]
#num_steps = 1000000
num_steps = 100000
num_repeats = 2
wlb_dg = wlbe.WL_Boltzmann_data_generator(dim, Ts, num_steps, num_repeats)
wlb_dg.run()
Nb: The WangLandau algorithm usually yields extreme answers when very few points are used (compared to the number of points needed for convergence) since it works in log scale. We can see this behavior here since we get a warning about invalid numerical values when trying to compute the median error. This is clearly visible in the plot itself as the error curves are missing values for steps < 10^4.
wlbe.WL_Boltzmann_plotter.fig_relative_std_dev("odir",dim)
wlbe.WL_Boltzmann_plotter.fig_error_wrt_Lebesgue("odir",dim)
wlbe.WL_Boltzmann_plotter.fig_climbing_times("odir",dim)
wlbe.WL_Boltzmann_plotter.fig_condition_number("odir",dim)
wlbe.WL_Boltzmann_plotter.fig_ATMs("odir",dim)