"""Module for building atomic structures"""

from ASE import ListOfAtoms, Atom
from math import sqrt

def FCC100(symbol, a, layers, h):
    """Build a fcc(100) surface

    symbol: chmical symbol ('H', 'Li', ...)
    a     : lattice constant
    layers: number of layers
    h     : vacuum of unit cell"""


    # Distance between layers:
    dz = a / 2

    # height of the unit cell (= length of a3)
    c = layers*dz + h

    # Unit Cell parameters:
    a1=a*sqrt(2.0)/2.0

    c=layers*dz+h
   
    # Start with an empty ListOfAtoms object:
    atoms = ListOfAtoms([], cell=[(a1, 0, 0),(0, a1, 0),(0, 0, c)])

    # Fill in the atoms using cartesian coordinates:
    for n in range(layers):
        z=-n*dz
        if (n%2)==0: 
           x=0.0
           y=0.0
        else:
           x=0.5*a1
           y=0.5*a1
        atoms.append(Atom(symbol, (x,y,z)))
   
    return atoms

if __name__ == '__main__':
    a = FCC100('Al', 4.05, 4, 20.0)
    from ASE.Visualization.xmakemol import *
    xmakemol(a, repeat=(3, 3, 2))