# Defining your own potential class#

## Introduction#

There are two ways to define a new potential class: with pure-Python, or with C and Cython. The advantage to writing a new class in Cython is that the computations can execute with C-like speeds, however only certain integrators support using this functionality (Leapfrog and DOP853) and it is a bit more complicated to set up the code to build the C+Cython code properly. If you are not familiar with Cython, you probably want to stick to a pure Python class for initial testing. If there is a potential class that you think should be included as a built-in Cython potential, feel free to suggest the new addition as a GitHub issue!

For the examples below the following imports have already been executed:

```
>>> import numpy as np
>>> import gala.potential as gp
>>> import gala.dynamics as gd
```

### Implementing a new potential with Python#

New Python potentials are implemented by subclassing
`PotentialBase`

and defining functions that
compute (at minimum) the energy and gradient of the potential. We will work
through an example below for adding the Henon-Heiles potential.

The expression for the potential is:

With this parametrization, there is only one free parameter (`A`

), and the
potential is two-dimensional.

At minimum, the subclass must implement the following methods:

`__init__()`

`_energy()`

`_gradient()`

The `_energy()`

method should compute the potential energy at a given position
and time. The `_gradient()`

method should compute the gradient of the
potential. Both of these methods must accept two arguments: a position, and a
time. These internal methods are then called by the
`PotentialBase`

superclass methods
`energy()`

and
`gradient()`

. The superclass methods
convert the input position to an array in the unit system of the potential for
fast evaluation. The input to these superclass methods can be
`Quantity`

objects,
`PhaseSpacePosition`

objects, or `ndarray`

.

Because this potential has a parameter, the `__init__`

method must accept
a parameter argument and store this in the `parameters`

dictionary attribute
(a required attribute of any subclass). Let’s write it out, then work through
what each piece means in detail:

```
>>> class CustomHenonHeilesPotential(gp.PotentialBase):
... A = gp.PotentialParameter("A")
... ndim = 2
...
... def _energy(self, xy, t):
... A = self.parameters['A'].value
... x,y = xy.T
... return 0.5*(x**2 + y**2) + A*(x**2*y - y**3/3)
...
... def _gradient(self, xy, t):
... A = self.parameters['A'].value
... x,y = xy.T
...
... grad = np.zeros_like(xy)
... grad[:,0] = x + 2*A*x*y
... grad[:,1] = y + A*(x**2 - y**2)
... return grad
```

The internal energy and gradient methods compute the numerical value and
gradient of the potential. The `__init__`

method must take a single argument,
`A`

, and store this to a parameter dictionary. The expected shape of the
position array (`xy`

) passed to the internal `_energy()`

and `_gradient()`

methods is always 2-dimensional with shape `(n_points, n_dim)`

where
`n_points >= 1`

and `n_dim`

must match the dimensionality of the potential
specified in the initializer. Note that this is different from the shape
expected when calling the public methods `energy()`

and `gradient()`

!

Let’s now create an instance of the class and see how it works. For now, let’s
pass in `None`

for the unit system to designate that we’ll work with
dimensionless quantities:

```
>>> pot = CustomHenonHeilesPotential(A=1., units=None)
```

That’s it! We now have a potential object with all of the same functionality as the built-in potential classes. For example, we can integrate an orbit in this potential (but note that this potential is two-dimensional, so we only have to specify four coordinate values):

```
>>> w0 = gd.PhaseSpacePosition(pos=[0., 0.3],
... vel=[0.38, 0.])
>>> orbit = gp.Hamiltonian(pot).integrate_orbit(w0, dt=0.05, n_steps=10000)
>>> fig = orbit.plot(marker=',', linestyle='none', alpha=0.5)
```

(Source code, png, pdf)

We could also, for example, create a contour plot of equipotentials:

```
>>> grid = np.linspace(-1., 1., 100)
>>> from matplotlib import colors
>>> import matplotlib.pyplot as plt
>>> fig, ax = plt.subplots(1, 1, figsize=(5,5))
>>> fig = pot.plot_contours(grid=(grid, grid),
... levels=np.logspace(-3, 1, 10),
... norm=colors.LogNorm(),
... cmap='Blues', ax=ax)
```

(Source code, png, pdf)

### Adding a custom potential with Cython#

Adding a new Cython potential class is a little more involved as it requires writing C-code and setting it up properly to compile when the code is built. For this example, we’ll work through how to define a new C-implemented potential class representation of a Keplerian (point-mass) potential. Because this example requires using Cython to build code, we provide a separate demo GitHub repository with an implementation of this potential with a demonstration of a build system that successfully sets up the code.

New Cython potentials are implemented by subclassing
`CPotentialBase`

, subclassing
`CPotentialWrapper`

, and defining C functions
that compute (at minimum) the energy and gradient of the potential. This
requires creating (at minimum) a Cython file (.pyx), a C header file (.h), and a
C source file (.c).