Nested sampling method

Authors:

Noam, Rob, Gabor, Livia

Date:

2015

Overview

The nested sampling method was first introduced by John Skilling [1] in the field of applied probability and inference to efficiently sample probability densities in high-dimensional spaces where the regions contributing most of the probability mass are exponentially localized. It is an annealing algorithm that creates a sequence of probability distributions, each more concentrated than the previous one near the high-likelihood region of the parameter space - i.e. in the context of materials, the low-energy region of configuration space.

Since its original inception, nested sampling has also been applied to atomistic systems, and its several advantages mean it became a powerful method to sample atomic configuration spaces.[#f2]_ * calculate the partition function, hence all thermodynamic properties become accessible

  • calculate the heat capacity and locate the phase transitions

  • with an order parameter calculate the free energy

  • the sampling process itself is independent of temperature, thus the calculation of thermodynamic quantities is a simple post-processing step

  • no prior knowledge is needed of the phases or phase transitions

  • can be used with both constant volume and constant pressure calculations

  • calculate the entire phase diagram in an automated way

  • considerable computational gain over parallel tempering

Publications

L.B. Partay, A.P. Bartok, G. Csanyi, Efficient Sampling of Atomic Configurational Spaces, J. Phys. Chem. B (2010), 114, 10502–10512, https://pubs.acs.org/doi/abs/10.1021/jp1012973

R.J.N. Baldock, L.B. Partay, A.P. Bartok, M.C. Payne, G. Csanyi, Determining pressure-temperature phase diagrams of materials, Phys. Rev. B (2016), 93, 174108, https://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.174108

R.J.N. Baldock, N. Bernstein, K. M. Salerno, L.B. Partay, G. Csanyi, Constant-pressure nested sampling with atomistic dynamics, Phys. Rev. E (2017), 96, 043311, https://link.aps.org/doi/10.1103/PhysRevE.96.043311

L.B. Partay, G. Csanyi and N. Bernstein Nested sampling for materials, Eur. Phys. J. B (2021) 94, 159, https://link.springer.com/article/10.1140/epjb/s10051-021-00172-1

Further examples of publications using the pymatnest package:

L.B. Partay On the performance of interatomic potential models of iron: Comparison of the phase diagrams, Comp. Mat. Sci. (2018), 149, 153, https://www.sciencedirect.com/science/article/pii/S0927025618301794

J. Dorrell, L.B. Partay Thermodynamics and the potential energy landscape: case study of small water clusters, Phys. Chem. Chem. Phys. (2019), 21, 7305 https://pubs.rsc.org/en/content/articlehtml/2019/cp/c9cp00474b

A.P. Bartok, G. Hantal and L.B. Partay Insight into Liquid Polymorphism from the Complex Phase Behavior of a Simple Model, Phys. Rev. Lett. (2021) 127, 015701 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.015701

L.B. Partay and G. Hantal Stability of the high-density Jagla liquid in 2D: sensitivity to parameterisation, Soft Matter, (2022) 18, 5261 https://pubs.rsc.org/en/content/articlelanding/2022/SM/D2SM00491G

Algorithm

Nested sampling is an iterative, “top-down” method, it samples the Potential Energy Landscape (PES) through a series of nested energy levels, starting from the high energy region (gas phase) and going towards the global minimum. The sampling is done using a set of walkers - these are randomly generated configurations at the initial step of the sampling, and the number of these determine the “resolution” of which the PES is sampled. Then, at every iteration step, the walker with the highest energy (enthalpy in case of constant pressure sampling) is removed, and a new one is generated to keep the number of walkers constant. To generate the new configuration, one of the existing walkers is randomly selected and cloned, and this cloned configuration is modified (through an MCMC, a total-enthalpy Hamiltonian MC or a Galilean MC walk), such that its energy (enthalpy) does not exceed the that of the last removed walker. The iteration is continued until a satisfactory low energy configuration is reached.

_images/NS_PES_animation.gif

pymatnest

The pymatnest package is a software library written in Fortran 95/python for the purpose of carrying out nested sampling calculations with a variety of options suitable for different systems. It can be used with the supplied fortran potential models and it also has interfaces with the following packages:

  • LAMMPS

MC and MD step algorithms

rej_free_perturb_velo

  • if atom_velo_rej_free_fully_randomize, pick random velocities consistent with Emax

  • else perturb velocities

    • if current velocity=0, can’t rescale, so pick random velocities consistent with Emax

    • else, pick new random magnitude consistent with Emax, random rotation of current direction with angle uniform in +/- atom_velo_rej_free_perturb_angle

do\_MC\_atom\_velo\_walk
  • Else if MC_atom_velo_walk_rej_free

    • call rej_free_perturb_velo()

  • else do MC pertubation to velocities

do_MD_atom_walk

  • if MD_atom_velo_pre_perturb, call do_MC_atom_velo_walk() for magnitude and rotation

  • propagate in time atom_traj_len time steps of length MD_atom_timestep

  • If MD_atom_reject_energy_violation is set, accept/reject entire move on E deviating by less than MD_atom_energy_fuzz times kinetic energy

  • accept/reject entire move on E < Emax and KE < KEmax

  • if reject

    • set positions, velocities, energy back to value before perturbation (maybe should be after?)

  • else

    • flip velocities if MD_atom_velo_flip_accept

  • if MD_atom_velo_post_perturb, call do_MC_atom_velo_walk() for magnitude and rotation

do_MC_atom_walk

  • if MC_atom_velocities and MC_atom_velocities_pre_perturb, call do_MC_atom_velo_walk() to perturb velocities, magnitude and and rotation

  • if using fortran calculator and not reproducible

    • call fortran MC code f_MC_MD.MC_atom_walk

  • else

    • do python MC

    • if MC_atom_Galilean

      • go Galilean MC in python

    • else
      • loop atom_traj_len times
        • loop over atoms in random order

          • propose single atom move

          • accept/reject on E < Emax

do_MC_swap_step

  • return if all atomic numbers are identical

  • randomly pick a desired cluster size

  • pick two clusters with distinct atomic numbers, backing off to smaller clusters on failure to find appropriate larger clusters, but always pick at least a pair of atoms to swap

  • accept swap if energy < Emax

do_MC_cell_volume_step

do_MC_cell_shear_step

do_MC_cell_stretch_step

do_atom_walk

  • loop n_atom_steps_per_call times, calling do_MC_atom_walk() or do_MD_atom_walk()

walk_single_walker
  • create list

    • do_atom_walk

      n_atom_step_n_calls

    • do_cell_volume_step

      n_cell_volume_steps

    • do_cell_shear_step

      n_cell_shear_steps

    • do_cell_stretch_step

      n_cell_stretch_steps

    • do_swap_step

      n_swap_steps

    • do_semi_grand_step

      n_semi_grand_steps

  • loop while n_model_calls_used < n_model_calls

    • pick random item from list

    • do move

  • perturb final energy by random_energy_perturbation

full_auto_set_stepsizes

  • Step sizes for each (H)MC move are set via a loop which performs additional exploration moves, calibrating each step size to obtain an acceptance rate inside a specified range.

  • The routine is MPI parallelised, so that the wall time goes as 1/num_of_processes

  • For each (H)MC move type the following is performed

    • Set ‘’movement_args’’ parameters so that only one (H)MC call is made at a time

    • Min and max acceptance rates are copied from parameters MC_adjust_min_rate / MD_adjust_min_rate and MC_adjust_max_rate / MD_adjust_max_rate

    • Step size calibration loop:
      • Repeat the following 200/num_of_MPI_processes times:

        • Copy a configuration from the live set (each MPI process chooses a different configuration)

        • Each MPI processes performs one (H)MC move on its cloned configuration running statistics for the number of accepted/rejected moves on each process are recorded

      • The total number of accepted/rejected moves for this step size (summed across all MPI processes) are estabilshed
        • If the total acceptance rate is within the desired range, return this stepsize
          • If this is NOT the first time the loop has been performed for this (H)MC move AND we previously obtained an acceptance rate on one side of the desired range, and now find an acceptance rate on the other side of the desired range

            • Return the step size that gave an acceptance rate closest to the middle of the desired range.

        • Store step length and acceptance rate

        • update step length, by

          or by MC_adjust_step_factor, to improve acceptance rate

        • Check that step size is not larger than max allowed value (specified by user), and also that step size is not smaller than 10-20(useful for detecting errors).

  • Return step sizes and time taken for routine to run

main: parse arguments
  • process n_atom_steps
    • If break_up_atom_traj

      • n_atom_steps_per_call = 1

      • n_atom_steps_n_calls = n_atom_steps

    • else

      • n_atom_steps_per_call = n_atom_steps

      • n_atom_steps_n_calls = 1