[yt-users] creating gradient of a field as a derived field

[turhan nasri]

Hi yt people,

I need a little help creating a derived field which will return me the
negative gradient of gravitational potential


*@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 the right method to define the field. thanks

Best
Turhan
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