Suspected Typo Fix in pymatgen.io.vasp.optics

[kavanase]

Hi!
Firstly, thanks to the developers for the recent addition of the pymatgen.io.vasp.optics module, it's been very very useful! 🙌

This is just a small fix for a suspected typo in handling ismear in the delta_func() and step_func() functions. ISMEAR is zero for Gaussian smearing, and indeed in the docstrings for get_epsilon() it states: ismear: Smearing method (only has 0:gaussian, >0:Methfessel-Paxton). However, in the current version of the code:

def delta_func(x, ismear):
    """Replication of VASP's delta function"""
    if ismear < -1:
        raise ValueError("Delta function not implemented for ismear < -1")
    if ismear == -1:
        return step_func(x, -1) * (1 - step_func(x, -1))
    if ismear < 0:
        return np.exp(-(x * x)) / np.sqrt(np.pi)
    return delta_methfessel_paxton(x, ismear)

ismear = 0 ends up returning the Methfessel-Paxton smearing (delta_methfessel_paxton), however the function under if ismear < 0 is the Gaussian smearing. ismear should be an integer anyway, so having ismear < -1 raise an error and ismear == -1 return something else, it means that the ismear < 0 line is always skipped. This is also the case for the step_func() function.

I believe this is likely a small typo, and should instead by if ismear == 0:. I have changed it to this in this PR, and tests are passing.

[janosh]

Thanks for the fix! 👍 This sounds sensible. We should definitely have tests for these functions.