Welcome to open-moldyn’s documentation!

Presentation

Open-moldyn is the result of an internship project at the IPR (Institut de Physique de Rennes) aiming at creating an (hopefully) fast and easy to use 2D molecular dynamics simulation tool with python.

This tool features a GUI (graphical interface) and is meant to be installed with pip (as every respectable python packages should be).

Some links bellow:

• Github repository : https://github.com/open-moldyn/moldyn

• PyPI depot : https://pypi.org/project/open-moldyn/

Simulation

Model builder

Model builder. Stores and defines physical properties of a group of atoms.

The model handles two species of atom (one of them can be ignored by setting the correct mole fraction). The first species values are stored in the first part of arrays (lower indices), the second species in what lasts (higher indices). This is meant to facilitate computation of inter-atomic forces and potential energy.

class moldyn.simulation.builder.Model(pos=None, v=None, npart=0, x_a=1.0)

Parameters

• pos (np.array) –

  Atoms’ position.

  Note

  If v is not set, it is initialized as an array full of zeros.

• v (np.array) –

  Atoms’ speed.

  Note

  Taken in account only if pos is set.

• npart (int) –

  Total number of atoms.

  Note

  Setting pos overrides this.

• x_a (float) – Mole fraction of species A.

T

Temperature. Is calculated from the average kinetic energy of the atoms. May be set to any positive value, in which case atoms’ speed will be scaled to match the desired temperature.

Type

float

EC

Microscopic kinetic energy.

Note

Cannot be changed as-is, but setting T is one way to do so.

Type

float

total_EC

Total kinetic energy.

Note

Cannot be changed as-is.

Type

float

kB

Boltzmann constant. If changed, will affect the way the model behaves regarding temperature.

Type

float

pos

v

List of atom positions and speeds. First axis represents the atom, second axis the dimension (x or y).

Type

np.array

dt

Timestep used for simulation.

Note

An acceptable value is calculated when species are defined, but it may be set to anything else.

Type

float

npart

Total number of atoms.

Type

int

x_a

Mole fraction of species A.

Type

float

n_a

Atom number for species A, calculated from x_a and npart If set, x_a will be recalculated.

Type

int

epsilon_a

epsilon_b

Epsilon value (J, in Lennard-Jones potential) for species a or b.

Type

float

sigma_a

sigma_b

Sigma value (m, in Lennard-Jones potential) for species a or b.

Type

float

epsilon_ab

sigma_ab

Inter-species epsilon and sigma values.

Note

Cannot be changed as-is. If you want to change these values, modify the corresponding items in the params dictionary.

Type

float

re_a

re_b

re_ab

Estimated radius of minimum potential energy.

Type

float

rcut_fact

When the model is simulated, atoms further than rcut_fact*re do not interact. Defaults to 2.0.

Type

float

params

Model parameters, needed for the simulation.

Warning

Changing directly these values may lead to unpredicted behaviour if not documented.

Type

dict

kong

Kong rules to estimate inter-species sigma and epsilon parameters.

Type

dict

inter_species_rule

Rules to automatically estimate inter-species sigma and epsilon parameters. Defaults to kong.

Type

dict

x_lim_inf

y_lim_inf

Lower x and y position of the box boundaries.

Type

float

x_lim_sup

y_lim_sup

Upper x and y position of the box boundaries.

Type

float

length_x

length_y

Size of the box along x and y axis. Those parameter are tied with *_lim_*** and any change on one of them is correctly taken in account.

Type

float

lim_inf

lim_sup

length

2 elements wide array containing corresponding (x_*, y_*) values.

Note

Cannot be changed as-is.

Type

np.array

x_periodic

y_periodic

Defines periodic conditions for x and y axis. Set to 1 to define periodic boundary condition or 0 to live in an infinite empty space. Defaults to 0.

Type

int

mass

Total mass in the model.

Note

Cannot be changed as-is.

Type

float

m

Mass of each atom. Shape is (npart, 2) in order to facilitate calculations of kinetic energy and Newton’s second law.

Warning

You should not change those values unless you know what you are doing.

Type

np.array

_m()

Constructs m.

Returns

m

Return type

np.array

atom_grid(n_x, n_y, d)

Creates a grid containing n_x*n_y atoms.

Sets npart, pos, dt, v and m.

Parameters

• n_x (int) – number of columns

• n_y (int) – number of rows

• d (float) – inter-atomic distance

copy()

Returns

Copy of the current model

Return type

builder.Model

random_speed()

Gives a random speed to the atoms, following a normal law, in order to have a strictly positive temperature.

set_a(epsilon, sigma, m)

Sets species a parameters.

Parameters

• epsilon (float) – Epsilon in Lennard-Jones potential.

• sigma (float) – Sigma in Lennard-Jones potential.

• m (float) – Mass of the atom.

set_ab(a, b)

Sets species a and b parameters, and calculates inter-species parameters.

Parameters

• a (tuple) – First species parameters, under the form (epsilon, sigma, mass)

• b (tuple) – Second species parameters, under the same form.

set_b(epsilon, sigma, m)

Same as set_a.

set_dt(dt=None)

Defines the timestep used for simulation.

Parameters

dt (float) – Desired timestep. If not set, will be calculated from species a’s properties.

set_periodic_boundary(x=1, y=1)

Set periodic boundaries on both axis.

Parameters

• x (int) – see x_periodic

• y (int) – see y_periodic

shuffle_atoms()

Shuffle atoms’ position in order to easily create a homogeneous repartition of the two species. Should be called just right after the positions are defined.

Note

Atoms’ speed is not shuffled.

Simulator

Simulator. Simulates the dynamics of a model (on the CPU or GPU).

class moldyn.simulation.runner.Simulation(model=None, simulation=None, prefer_gpu=True)

Simulator for a model.

Parameters

• model (builder.Model) – Model to simulate. The original model object is copied, and thus preserved, which allows it to serve as a reference.

• simulation (Simulation) – Simulation to copy. The simulation is copied, and the computing module reinitialized.

• prefer_gpu (bool) – Specifies if GPU should be used to compute inter-atomic forces. Defaults to True, as it generally results in a significant speed gain.

model

Model that is simulated.

Warning

Works as usual, but changing its properties will not affect the simulation as intended. You should construct another simulation from this model to correctly take in account the changes. This is due to the fact that some values (eg. Lennard-Jones parameters) are treated as constants during shader initialisation, in order to speed up the calculations.

Type

builder.Model

current_iter

Number of iterations already computed, since initialisation.

Type

int

context

ModernGL context used to build and run compute shader.

Type

moderngl.Context

F

Last computed forces applied to atoms. Initialized to zeros.

Warning

Changing the values will affect behavior of the model.

Type

numpy.ndarray

F_f(t)

Parameters

t – Time in seconds.

Returns

External forces applied at time t (2-component vector).

Return type

numpy.ndarray

iter(n=1, callback=None)

Iterates one or more simulation steps.

Basically, follows the Position-Verlet method to update positions and speeds, and computes inter-atomic forces derived from Lennard-Jones potential.

Parameters

• n (int) – Number of iterations to perform.

• callback (callable) – A callback function that must take the Simulation object as first argument. It is called at the end of each iteration.

Note

Setting n is significantly faster than calling iter() several times.

Example

model.iter(5)

set_Fx_ramps(t, Fx)

Creates a function based on ramps and uses it for external forces control along axis x. See set_T_ramps for further details.

Parameters

• t (array) – Time.

• Fx (array) – Associated forces.

set_Fy_ramps(t, Fy)

Creates a function based on ramps and uses it for external forces control along axis y. See set_T_ramps for further details.

Parameters

• t (array) – Time.

• Fy (array) – Associated forces.

set_T_f(f)

Sets function that controls temperature.

Parameters

f (callable) – Must take time (float) as an argument and return temperature (in K, float).

set_T_ramps(t, T)

Creates a function based on ramps and uses it for temperature control. Values of the function are interpolated between points given in t and T. Temperature is supposed constant before the first point and after the last one.

Parameters

• t (array) – Time.

• T (array) – Associated temperatures.

Inter-atomic compute modules

class moldyn.simulation.forces_CPU.ForcesComputeCPU(consts, compute_npart=None, compute_offset=0)

Compute module. Runs on CPU. Uses numba for JIT compilation and multiprocessing for multicore. See ForcesComputeGPU for documentation.

class moldyn.simulation.forces_GPU.ForcesComputeGPU(consts, compute_npart=None)

Compute module. Runs on GPU.

Parameters

consts (dict) – Dictionary containing constants used for calculations.

npart

Number of atoms.

Type

int

consts

Dictionary containing constants used for calculations, and some parameters to run the compute shader.

Type

dict

get_COUNT()

Returns

Near atoms (one could count this as bonds).

Return type

np.ndarray

get_F()

Returns

Computed inter-atomic forces.

Return type

np.ndarray

get_PE()

Returns

Computed potential energy.

Return type

np.ndarray

set_pos(pos)

Set position array and start computing forces.

Parameters

pos (np.ndarray) – Array of positions.

Processing

Data processing

moldyn.processing.data_proc.PDF(pos, nb_samples, rcut, bin_count)

Pair Distribution Function. Returns normalized histogram of distance between atoms.

Parameters

• pos (np.array) – Array containing atoms position

• nb_samples (int) – Number of atoms from which to generate the histogram

• rcut (number) – Maximum distance to consider

• bin_count (int) – Number of bins of the histogram

Returns

bins, hist – bins being the distances, hist the normalized (regarding radius) histogram

Return type

tuple(np.array, np.array)

moldyn.processing.data_proc.cached(f, _cache={<function PDF>: numexpr.utils.CacheDict, <function density>: numexpr.utils.CacheDict, <function compute_strain>: numexpr.utils.CacheDict})

Parameters

f –

moldyn.processing.data_proc.compute_strain(model0, model1, rcut)

Compute the local deformation tensor for each atom.

It will try to use GPU but will fallback on CPU if not available

Parameters

• model0 (simulation.builder.Model) – The model at time t

• model1 (simulation.builder.Model) – The model at time t-dt

• rcut (float) –

Returns

• A vector containing the 2D deformation tensor of each atom

• (in the order of model.pos).

Note

Due to numerical calculation imprecision the deformation tensor may not be quantitatively accurate (or even symmetrical).

moldyn.processing.data_proc.density(model, refinement=0)

Create a Voronoi mesh and calculate the local particle density on its vertices.

The local density is calculated as follows: for each vertex, compute the density of each neighbour region as one over the area and assign the average of the neighbouring density to the vertex.

Parameters

• model (simulation.builder.Model) – the Model object containing

• refinement (int (defaults : 0)) – number of subdivision for refining the mesh (0 == None)

Returns

• tri (matplotlib.tri.Triangulation) – the triangulation mesh (refined if set as)

• vert_density (numpy.array) – the array containing the local denstity associated with the tri mesh

Example

To plot the result using matplotlib use :

import matplotlib.pyplot as plt
tri, density = data_proc.density(model)
plt.tricontour(tri, density) # to draw contours
plt.tricontourf(tri, density) # ot draw filled contours
plt.show()

Note

As of now, the numerical results may not be quantitatively accurate but should qualitatively represent the density.

Visualization

moldyn.processing.visualisation.make_movie(simulation, dynstate, name, pfilm=5, fps=24, callback=None)

Makes a .mp4 movie of simulation from the history saved in dynstate.

Parameters

• simulation (Simulation) – The simulation object

• dynstate (DynState) – The dynState object containing the position history file.

• name (str) – The path (and name) of the file to be writen.

• pfilm (int) – To make the movie, takes every pfilm position.

• fps (int) – The number of frame per seconds of the film.

• callback (function) – An optional callback function that is called every time a frame is made and is passed the current iteration number.

moldyn.processing.visualisation.plot_density(model, levels=None, refinement=0)

Compute and plot the contours of the local density of model.

Parameters

• model (Model) – The model to plot

• levels (int or array-like) – the number of levels or a sorted array-like object of levels

• refinement (int) – the level of refinement

moldyn.processing.visualisation.plot_density_surf(model, refinement=0)

Compute and plot the 3D surface of the local density of model.

Parameters

• model (Model) – The model to plot.

• refinement (int) – the level of refinement

moldyn.processing.visualisation.plot_densityf(model, levels=None, refinement=0)

Compute and plot the filled contours of the local density of model.

Parameters

• model (Model) – The model to plot

• levels (int or array-like) – the number of levels or a sorted array-like object of levels

• refinement (int) – the level of refinement

moldyn.processing.visualisation.plot_particles(model)

Plot the position stored in model.

Parameters

model (Model) – The model to plot

Utils

Utility module

Data management

class moldyn.utils.data_mng.DynState(treant, *, extraction_path='/home/docs/.local/share/open-moldyn/tmp')

A Treant specialized for handling .npy and .json files for moldyn

POS

standard name of the position file (“pos.npy”)

Type

str

POS_H

standard name of the position history file (“pos_history.npy”)

Type

str

VEL

standard name of the velocity file (“velocities.npy”)

Type

str

STATE_FCT

standard name of the state function file (“state_fct.json”)

Type

str

PAR

standard name of the parameter file (“parameters.json”)

Type

str

open(file, mode='r')

Open the file in this tree (ie. directory and subdir). Return the appropriate IO class depending of the type of the file.

If the type is not recognize, it opens the file and return the BytesIO or StringIO object.

Parameters

• file (str) – The name of the file. This file must be in this tree (ie. directory and subdir).

• mode (str (default='r')) –

  The mode with which to open the file :

  • ’r’ to read

  • ’w’ to write

  • ’a’ to append

  • ’b’ to read or write bytes (eg. ‘w+b’ to write bytes)

Returns

• If file is a .npy file, return a NumpyIO object.

• If file is a .json file, return a ParamIO object.

• Else, return a StringIO or a BytesIO depending on mode.

Note

This method is designed to be used wih a context manager like this

t = DynState(dirpath)
# here t.open returns a NumpyIO object as the file is .npy
with t.open("pos.npy", 'r') as IO:
    arr = IO.load() #load an array
with t.open("pos.npy", 'w') as IO:
    IO.save(arr) #save an array

save_model(model)

Save the positions, the velocities and the parameters of the model.

The position and velocity arrays are saved as numpy files and the parameter dictionary as a .json file

Parameters

model (simulation.builder.Model) – The model to be saved.

to_zip(path)

Zip every leaf (aka. file) of the dynState treant into an archive at path.

Parameters

path (str) – The path of the archive.

class moldyn.utils.data_mng.NumpyIO(dynState, file, mode)

An interface to interact with numpy save files with a context manager.

dynState

The treant that support the leaf (file) associated with it.

Type

datreant.Treant

mode

The access mode of the file (usually ‘r’ or ‘w’ or ‘a’)

Type

str

file_name

The file name of the .npy file associated with it.

Type

datreant.Leaf

file

the file that is opened (contains None until entering a context manager)

Type

file object

Example

t = DynState(dirpath)
# here t.open returns a NumpyIO object as the file is .npy
with t.open("pos.npy", 'r') as IO:
    arr = IO.load() #load an array
with t.open("pos.npy", 'w') as IO:
    IO.save(arr) #save an array

close()

Close the internal file.

load()

Load an array stored in the file.

Returns

arr – the array loaded

Return type

ndarray

open()

Open the internal file.

save(arr)

Save an array to the opened file.

Parameters

arr (ndarray (numpy)) – The array to be stored

class moldyn.utils.data_mng.ParamIO(dynState, file, **kwargs)

An interface to interact with json files as a dictionary.

Designed to be used with context manager (the with statement) and datreant. Upon entering the with statement it will attempt to load the json file at self.file_name into itself (as a dict). When leaving this context, it will store itself into the json file.

Inherits dict.

It will try to update the tags and categories of the treant dynState.

dynState

The treant that support the leaf (file) associated with it.

Type

datreant.Treant

file_name

The file name of the json file associated with it.

Type

datreant.Leaf

Example

t = DynState(dirpath)
# here t.open returns a ParamIO object as the file is .json
with t.open("params.json") as IO:
    IO["my_key"] = "my_value"
    random_var = IO[some_key]
# upon exiting the with block, the content of IO is stored
# in the .json file

Parameters

• dynState (Treant) –

• file (Leaf) –

• kwargs –

__enter__()

Try to load the content of the json file into itself

Try to open the json file from file_name and load the informtions it contains into itself. Will catch FileNotFoundError and JSONDecodeError

Returns

self

Return type

ParamIO

__exit__(exc_type, exc_val, exc_tb)

Open the json file file_name and dump itself into it.

Open the json file file_name and dump itself into it + update the tags and categories of dynState according to the CATEGORY_LIST of the module.

Parameters

• exc_type –

• exc_val –

• exc_tb –

from_dict(rdict)

Copy rdict into itself

Parameters

rdict (dict) – The remote dictionary from which to copy

to_attr(obj)

Dump the parameter dictionary in the object (obj) as attributes of said object.

Warning

Will change the value of each attribute of obj that have a name corresponding to a key of ParamIO.

Parameters

obj (an object) – The object to which it will dump its content as attributes

to_dict(rdict)

Copy itself into the remote dictionary :type rdict: dict :param rdict: The remot dictionary to which to copy :type rdict: dict

OpenGL utility tools

moldyn.utils.gl_util.source(uri, consts={})

Reads and replaces constants (in all caps) in a text file.

Parameters

• uri (str) – URI of the file to read.

• consts (dict) – Dictionary containing the values to replace.

Returns

File contents with replacements.

Return type

str

moldyn.utils.gl_util.testGL()

Tries to initialize a compute shader.

Returns

Initialisation success.

Return type

bool

Indices and tables

• Index

• Module Index

• Search Page

Credits

Open-moldyn is made by Alexandre Faye-Bedrin and Arthur Luciani (at the time students of the ENS Rennes and interns of the IPR) with the supervision of Yann Gueguen.

This package also includes datreant and appdirs to help managing data and files.