phonon bandstructure of phosphorene with ATK-Classical method

[beauyy]

Dear sir,
     I tried to calculate the phonon bandstructure of monolayer phosphorene with ATK-Classical method, but there were  negative bands. The bulk had been optimized to Max force less than 0.001 eV/A and Max stress less than 0.001 eV/A. How can I eliminate the negative bands?
# -------------------------------------------------------------
# Bulk configuration
# -------------------------------------------------------------

# Set up lattice
vector_a = [6.68209223283, 0.0, 0.0]*Angstrom
vector_b = [0.0, 9.16633794912, 0.0]*Angstrom
vector_c = [0.0, 0.0, 40.0]*Angstrom
lattice = UnitCell(vector_a, vector_b, vector_c)

# Define elements
elements = [Phosphorus, Phosphorus, Phosphorus, Phosphorus, Phosphorus,
            Phosphorus, Phosphorus, Phosphorus, Phosphorus, Phosphorus,
            Phosphorus, Phosphorus, Phosphorus, Phosphorus, Phosphorus,
            Phosphorus]

# Define coordinates
fractional_coordinates = [[ 0.24999996,  0.25000008,  0.44609425],
                          [ 0.        ,  0.41331253,  0.44609424],
                          [ 0.50000005,  0.4133125 ,  0.44609424],
                          [ 0.24999996,  0.74999997,  0.44609422],
                          [ 0.75      ,  0.74999994,  0.44609423],
                          [ 0.00000001,  0.91331268,  0.44609425],
                          [ 0.50000005,  0.91331264,  0.44609424],
                          [ 0.74999999,  0.25000003,  0.44609426],
                          [ 0.        ,  0.        ,  0.5       ],
                          [ 0.50000005,  0.00000003,  0.5       ],
                          [ 0.24999996,  0.16331255,  0.49999999],
                          [ 0.74999999,  0.1633126 ,  0.49999998],
                          [ 0.00000001,  0.49999985,  0.49999999],
                          [ 0.50000005,  0.49999989,  0.5       ],
                          [ 0.24999996,  0.66331266,  0.50000002],
                          [ 0.75      ,  0.66331269,  0.50000001]]

# Set up configuration
bulk_configuration = BulkConfiguration(
    bravais_lattice=lattice,
    elements=elements,
    fractional_coordinates=fractional_coordinates
    )

# -------------------------------------------------------------
# Calculator
# -------------------------------------------------------------

potentialSet = EAMFS_FeP_2004()
calculator = TremoloXCalculator(parameters=potentialSet)

bulk_configuration.setCalculator(calculator)
nlprint(bulk_configuration)
bulk_configuration.update()
nlsave('F:/P-As/pb-P.nc', bulk_configuration)

bulk_configuration = OptimizeGeometry(
        bulk_configuration,
        max_forces=0.001*eV/Ang,
        max_stress=0.001*eV/Ang**3,
        max_steps=200,
        max_step_length=0.2*Ang,
        trajectory_filename=None,
        optimizer_method=LBFGS(),
        )
nlsave('F:/P-As/pb-P.nc', bulk_configuration)
nlprint(bulk_configuration)

# -------------------------------------------------------------
# Phonon bandstructure
# -------------------------------------------------------------
phonon_bandstructure = PhononBandstructure(
    configuration=bulk_configuration,
    route=['G', 'X', 'Y', 'Z', 'G'],
    points_per_segment=100,
    number_of_bands=20
    )
nlsave('F:/P-As/pb-P.nc', bulk_configuration)
nlsave('F:/P-As/pb-P.nc', phonon_bandstructure)

[Anders Blom]

Probably this potential is not able to describe phosphorene. It's a potential for FeP, which is quite another thing.

[beauyy]

When I decreased the bulk of monolayer phosphorene into that contains 4 P atoms in a bulk, the negative bands vanshied. But the same phenomenon didn't happen in other 2D material, for example, Arsenene. Why?

[Jess Wellendorff]

I couldn't really say why this would happen one case but not in another, but in general you should always run your calculations in the most symmetry-reduced supercell as possible.