import numpy as np


def numb_atoms():
    return 1


def gen_box():
    return np.eye(3)


def poscar_unit(latt):
    box = gen_box()
    ret = ""
    ret += "SC : a = %f \n" % latt
    ret += "%.16f\n" % (latt)
    ret += "%.16f %.16f %.16f\n" % (box[0][0], box[0][1], box[0][2])
    ret += "%.16f %.16f %.16f\n" % (box[1][0], box[1][1], box[1][2])
    ret += "%.16f %.16f %.16f\n" % (box[2][0], box[2][1], box[2][2])
    ret += "Type\n"
    ret += "%d\n" % numb_atoms()
    ret += "Direct\n"
    ret += "%.16f %.16f %.16f\n" % (0.0, 0.0, 0.0)
    return ret
