creating gradient of a field as a derived field in project yt

Question

I need a little help creating a derived field which will return me the negative gradient of gravitational potential, hence the gravitational acceleration. I tried something like this:

@derived_field(name = "gradient_gravpot", units = " cm/s**2")
def _gradient (field, data):
        G = add_gradient_fields(("gas"," gravitational_potental"))
        return (- data["gravitational_potential_gradient_x"])

and I got this error

   NameError: global name 'add_gradient_fields' is not defined

please suggest me the right method to define the field. thanks

Answer 1

It looks like you got an answer on the yt project mailing list. That said, I'll repeat here that add_gradient_fields is a method of the Dataset class, so you will need to call it on a loaded dataset. For example:

ds = yt.load('path/to/dataset')
ds.add_gradient_fields(('gas', 'gravitational_potential'))