ASE defines a python class called Atom. It is the basic building block of a ListOfAtoms. From a python script, atoms are created like this:

>>> from ASE import Atom
>>> a1 = Atom('Si', (0, 0, 0))
>>> a2 = Atom('H', (1.3, 0, 0), mass=2)
>>> a3 = Atom(position=(0, 0, 0), Z=14)  # same is a1

The first argument to the constructor of an Atom object is the chemical symbol, and the second argument is the position. The position can be any numerical sequence of length three. The properties of an atom can also be set using keywords like it is done in the a2 example. The following keywords can be used:

keyword description type default value method name
symbol chemical symbol str   GetChemicalSymbol
position position seq (0, 0, 0) GetCartesianPosition
Z atomic number int   GetAtomicNumber
mass mass float   GetMass
tag tag int 0 GetTag
momentum momentum seq (0, 0, 0) GetCartesianMomentum
velocity velocity seq (0, 0, 0) GetCartesianVelocity
magmom magnetic moment float 0 GetMagneticMoment

seq: A sequence of three numbers.

One of the keywords symbol or Z must be given - other keywords have default values. The default value for the mass is set acording to symbol or Z:

>>> a1.GetMass()

Here we used one of the Get methods from the last column. An atom also has the corresponding Set methods, like SetCartesianVelocity and SetMass:

>>> a1.SetCartesianVelocity((0, 0, 0.1))

There is also a method for calculating the kinetic energy of an atom:

>>> a1.GetKineticEnergy()

An atom also has two methods for interacting with a force calculator: GetCartesianForce() and _SetCartesianForce(force).

The ChemicalElements module

The ChemicalElements module defines an function Element() that can take an atomic number or a chemical symbol as argument:

>>> from ASE.ChemicalElements import Element
>>> e = Element('Fe')
>>> e.number
>>> e.mass
>>> Element(99).name

The Element() function returns a special object with the following attributes: symbol, number, name and, if it makes sence, also mass, covalent_radius, cpk_color and crystal_structure.

The individual properties are also avilable as lists:

>>> from import names
>>> names[5:11]
['Boron', 'Carbon', 'Nitrogen', 'Oxygen', 'Fluorine', 'Neon']