# pyretis.forcefield.potentials package¶

A collection of potential functions.

This package defines some potential functions. These potential functions can be used to create force fields.

## Package structure¶

### Modules¶

potentials.py (pyretis.forcefield.potentials.potentials)
This module defines some simple potential functions.

### Sub-packages¶

pairpotentials (pyretis.forcefield.potentials.pairpotentials)
This package defines different pair interactions, for instance the Lennard-Jones 6-12 simple cut potential.

### Important classes defined in this package¶

DoubleWell (DoubleWell)
A double well potential
RectangularWell (RectangularWell)
A rectangular well potential – useful as a bias potential.
PairLennardJonesCut (PairLennardJonesCut)
The Lennard-Jones potential in pure python.
PairLennardJonesCutnp (PairLennardJonesCutnp)
The Lennard-Jones potential, making use of numpy.
DoubleWellWCA (DoubleWellWCA)
A n-dimensional Double Well potential.

## pyretis.forcefield.potentials.potentials module¶

A collection of simple position dependent potentials.

This module defines some potential functions which are useful as simple models.

### Important classes defined here¶

DoubleWell (DoubleWell)
This class defines a one-dimensional double well potential.
RectangularWell (RectangularWell)
This class defines a one-dimensional rectangular well potential.
class pyretis.forcefield.potentials.potentials.DoubleWell(a=1.0, b=1.0, c=0.0, desc='1D double well potential')[source]

A 1D double well potential.

This class defines a one-dimensional double well potential. The potential energy () is given by

where is the position and , and are parameters for the potential. These parameters are stored as attributes of the class. Typically, both and are positive quantities, however, we do not explicitly check that here.

params

Contains the parameters. The keys are:

• a: The a parameter for the potential.
• b: The b parameter for the potential.
• c: The c parameter for the potential.

These keys corresponds to the parameters in the potential, .

Type: dict
__init__(a=1.0, b=1.0, c=0.0, desc='1D double well potential')[source]

Initialise the one dimensional double well potential.

Parameters: a (float, optional) – Parameter for the potential. b (float, optional) – Parameter for the potential. c (float, optional) – Parameter for the potential. desc (string, optional) – Description of the force field.
force(system)[source]

Evaluate forces for the 1D double well potential.

Parameters: system (object like System) – The system we evaluate the potential for. Here, we make use of the positions only. out[0] (numpy.array) – The calculated force. out[1] (numpy.array) – The virial, currently not implemented for this potential!
potential(system)[source]

Evaluate the potential for the one-dimensional double well.

Parameters: system (object like System) – The system we evaluate the potential for. Here, we make use of the positions only. out (float) – The potential energy.
potential_and_force(system)[source]

Evaluate the potential and the force.

Parameters: system (object like System) – The system we evaluate the potential for. Here, we make use of the positions only. out[0] (float) – The potential energy as a float. out[1] (numpy.array) – The force as a numpy.array of the same shape as the positions in particles.pos. out[2] (numpy.array) – The virial, currently not implemented for this potential!
class pyretis.forcefield.potentials.potentials.RectangularWell(left=0.0, right=1.0, largenumber=inf, desc='1D Rectangular well potential')[source]

A 1D rectangular well potential.

This class defines a one-dimensional rectangular well potential. The potential energy is zero within the potential well and infinite outside. The well is defined with a left and right boundary.

params

The parameters for the potential. The keys are:

• left: Left boundary of the potential.
• right: Right boundary of the potential.
• largenumber: Value of potential outside the boundaries.

It is possible to define left > right, however, a warning will be issued then.

Type: dict
__init__(left=0.0, right=1.0, largenumber=inf, desc='1D Rectangular well potential')[source]

Initialise the one-dimensional rectangular well.

Parameters: left (float, optional) – The left boundary of the potential. right (float, optional) – The right boundary of the potential. largenumber (float, optional) – The value of the potential outside (left, right). desc (string, optional) – Description of the force field.
check_parameters()[source]

Check the consistency of the parameters.

Returns: out (None) – Returns None but might give a warning.
potential(system)[source]

Evaluate the potential.

Parameters: system (object like System) – The system we evaluate the potential for. Here, we make use of the positions only. out (float) – The potential energy.