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Free Energy and Advanced Sampling Simulation Toolkit (FEASST)


The Free Energy and Advanced Sampling Simulation Toolkit (FEASST) is a free, open-source, public domain software to conduct molecular and particle-based simulations with Monte Carlo methods. FEASST can be used to simulate a wide variety of materials and physical phenomena including fluid phase equilibrium, self-assembly and aggregation of biological materials, colloids, polymers, ionic liquids and other complex fluids. FEASST supports many simulation methodologies, thermodynamic ensembles, interaction potentials and Monte Carlo trials with checkpointing and parallelization using input text files, a Python interface and a C++ interface. 


Logo for Free Energy and Advanced Sampling Simulation Toolkit (FEASST)

The FEASST logo uses the letter F to construct a cartoon of (blue) a low temperature free energy with a metastable and stable basin and (red) a flat histogram

Credit: NIST

Intended Impact

While flat-histogram Monte Carlo simulations have been shown to be a promising method for the direct calculation of complex fluid phase behavior with a variety of applications, most open-source Monte Carlo simulation packages do not support this method. We have therefore made FEASST available to the public to facilitate adoption by the broader simulation community. FEASST has been used in 20 or more publications so far, and continues to grow.


Make advanced sampling and flat histogram molecular simulation methods more accessible to the public.

Research Activities

FEASST has been continuously updated over many years, with plans to add more features in the future. A current summary of the techniques available at the time of this writing (FEASST v0.24.1 on 11/17/2023) is as follows:

Monte Carlo simulations techniques:

  • Metropolis
  • Wang-Landau
  • Transition-Matrix
  • Mayer-sampling

Thermodynamic ensembles:

  • Microcanonical ensemble
  • Canonical ensemble
  • Grand-canonical ensemble
  • Temperature and growth-expanded ensembles
  • Gibbs ensemble

Interaction potentials

  • Lennard-Jones and Mie with cut/force shift and corrections
  • Hard spheres and square wells
  • Patchy and anisotropic particles
  • Yukawa and charged interactions
  • Ewald summation and 2D slab correction
  • Bonds, angles and dihedrals
  • TraPPE small molecules and n-alkanes
  • Slab, cylindrical and spherical confinement
  • Cell list and neighbor list

Monte Carlo trials

  • Translation, rotation, crankshaft and pivot
  • Rigid cluster rotation and translation
  • Configurational bias transfers and partial regrowth
  • Dual-cut configurational bias
  • Aggregation volume bias
  • Reptation
  • Branches

Modern software

  • Multiple interfaces including: text input, C++ and a Python module
  • OpenMP parallelization and prefetching
  • Checkpoint files to save, restart and analyze simulations


  1. H. W. Hatch, N. A. Mahynski, V. K. Shen, “FEASST: Free Energy and Advanced Sampling Simulation Toolkit”, J Res Natl Inst Stan, 123:123004. (2018).
Created December 4, 2023, Updated December 5, 2023