.. _compositepotential:

************************************
Creating a multi-component potential
************************************

Potential objects can be combined into more complex *composite* potentials
using the :class:`~gala.potential.CompositePotential` or
:class:`~gala.potential.CCompositePotential` classes. These classes operate
like a Python dictionary in that each component potential must be named, and
the potentials can either be passed in to the initializer or added after the
composite potential container is already created.

For composing any of the built-in potentials or any external potentials
implemented in C, it is always faster to use
:class:`~gala.potential.CCompositePotential`, where the composition is done at
the C layer rather than in Python.

But with either class, interaction with the class is identical. Each component
potential must be instantiated before adding it to the composite potential::

    >>> import numpy as np
    >>> import gala.potential as gp
    >>> from gala.units import galactic
    >>> disk = gp.MiyamotoNagaiPotential(m=1E11, a=6.5, b=0.27, units=galactic)
    >>> bulge = gp.HernquistPotential(m=3E10, c=0.7, units=galactic)
    >>> pot = gp.CCompositePotential(disk=disk, bulge=bulge)

is equivalent to::

    >>> pot = gp.CCompositePotential()
    >>> pot['disk'] = disk
    >>> pot['bulge'] = bulge

In detail, the composite potential classes subclass
:class:`~collections.OrderedDict`, so in this sense there is a slight difference
between the two examples above. By defining components after creating the
instance, the order is preserved. In the above example, the disk potential
would always be called first and the bulge would always be called second.

The resulting potential object has all of the same properties as individual
potential objects::

    >>> pot.value([1.,-1.,0.]) # doctest: +FLOAT_CMP
    <Quantity [-0.12891172] kpc2 / Myr2>
    >>> pot.acceleration([1.,-1.,0.]) # doctest: +FLOAT_CMP
    <Quantity [[-0.02271507],
               [ 0.02271507],
               [-0.        ]] kpc / Myr2>
    >>> grid = np.linspace(-3.,3.,100)
    >>> fig = pot.plot_contours(grid=(grid,0,grid))

.. plot::
    :align: center

    import numpy as np
    import gala.dynamics as gd
    import gala.potential as gp
    from gala.units import galactic

    disk = gp.MiyamotoNagaiPotential(m=1E11, a=6.5, b=0.27, units=galactic)
    bulge = gp.HernquistPotential(m=3E10, c=0.7, units=galactic)
    pot = gp.CompositePotential(disk=disk, bulge=bulge)

    grid = np.linspace(-3.,3.,100)
    fig = pot.plot_contours(grid=(grid,0,grid))