from ase import Atoms
from ase.io import read, write

atoms = Atoms('HH', [[.0,.0,.0], [.0,.0,.74]], pbc=True, cell=[5, 5, 5])
atoms.set_initial_magnetic_moments([1, -1])
moms = atoms.get_initial_magnetic_moments()
write('test.traj',atoms)
atoms = read('test.traj')
assert (atoms.get_initial_magnetic_moments() == moms).all()