### ListOfAtoms

A ListOfAtoms object is a collection of atoms. The ASE implementation of a ListOfAtoms can be used like this:

```>>> from ASE import Atom, ListOfAtoms
>>> m = ListOfAtoms([Atom('C', (0, 0, 0)),
...                  Atom('O', (0, 0, 1.0))])
```

The first argument to the ListOfAtoms constructor must be a python list of atoms. There are two optional keyword arguments:

cell: Unit cell size
This can be a sequence of three numbers for an orthorhombic unit cell or three by three numbers for a general unit cell (a sequence of three sequences of three numbers). The default value is (1, 1, 1).
periodic: Boundary conditions
The default value is False - a value of True would give periodic boundary conditions along all three axes. It is possible to give a sequence of three booleans to specify periodicity along specific axes.

Here is how you could make an infinite gold wire with a bond length of 2.9 �

```>>> from ASE import Atom, ListOfAtoms
>>> d = 2.9
>>> L = 10.0
>>> wire = ListOfAtoms([Atom('Au', (L / 2, L / 2, 0))],
...                    cell=(L, L, d), periodic=(0, 0, 1))
```

You can also use the following methods to work with the unit cell and the boundary conditions:

GetUnitCell():
Returns a three by three array.
SetUnitCell(cell, fix=False):
Change the size of the unit cell. If the optional argument fix is True (defaults to False), then the positons of the atoms are fixed, otherwise the atoms are moved so that their positions relative to the unit cell are kept.
GetBoundaryConditions():
Returns a tuple of three booleans.

Here is how you would do bulk ruthenium (hcp):

```>>> from math import sqrt
>>> a = 2.70
>>> c = 1.59 * a
>>> bulk = ListOfAtoms([Atom('Ru', (0, 0, 0)),
...                     Atom('Ru', (1 / 3., 1 / 3., 1 / 2.))],
...                    periodic=True)
>>> bulk.SetUnitCell([(a, 0, 0),
...                   (a / 2, a * sqrt(3) / 2, 0),
...                   (0, 0, c)])
```

In addition, an ListOfAtoms instance has the following methods:

GetKineticEnergy():
Returns the total kinetic energy.
Copy():
Return a fresh copy. Everything but a possibly attached calculator is copied (next section will explain how to attach a calculator).
Repeat(repeat):
The argument repeat is a sequence of three positive integers (n1, n2, n3 - one for each axis), and a copy of the ListOfAtoms object repeated n1 * n2 * n3 times is returned.

Warning

Using the same atom object in several ListOfAtoms objects is not allowed! Use a copy instead:

```>>> loa1.append(loa2)
RuntimeError: Atom belongs to another ListOfAtoms!
>>> loa1.append(loa2.Copy())
```

#### Array methods

It is possible to work with the properties of atoms in a ListOfAtoms by using the methods defined for the individual atoms like this:

```>>> m = ListOfAtoms([Atom('O', (0, 0, 0)),
...                  Atom('H', (0.773, 0.600, 0)),
...                  Atom('H', (-0.773, 0.600, 0))])
>>> m.GetCartesianPosition()
array([ 0.773,  0.6  ,  0.   ])
>>> m.SetCartesianPosition((0, -0.1, 0))
```

However, a ListOfAtoms that conforms to the ASE specification must have a set of array methods to do the same for all atoms in one step:

```>>> m.GetCartesianPositions()
array([[ 0.   ,  0.   ,  0.   ],
[ 0.773,  0.6  ,  0.   ],
[-0.773,  0.6  ,  0.   ]])
>>> m.SetCartesianPositions([(0, -0.1, 0),
...                          (0.773, 0.600, 0),
...                          (-0.773, 0.600, 0)])
```

The following methods must be defined:

Get methods Set methods type shape
GetCartesianPositions SetCartesianPositions Float (n, 3)
GetCartesianMomenta SetCartesianMomenta Float (n, 3)
GetCartesianVelocities SetCartesianVelocities Float (n, 3)
GetCartesianForces SetCartesianForces Float (n, 3)
GetTags SetTags Int (n,)
GetAtomicNumbers SetAtomicNumbers Int (n,)
GetMasses SetMasses Float (n,)
GetKineticEnergies   Float (n,)
GetGeneralizedPositions SetGeneralizedPositions Float (n*3,)
GetGeneralizedForces SetGeneralizedForces Float (n*3,)

The Get methods will return Numeric arrays of the given shape (n is the number of atoms) and type, and the Set methods will take anything with the correct shape.