def METHOD_NAME(params, x, y, x0, y0):
    """
    Calculate the values of an unrotated Gauss function 
    given positions in x and y in a mesh grid.
    Parameters
    ----------
    params : list
        List components : amplitude, x_stddev, y_stddev. 
        Acts as the inital guess from meta (params_guess).
    x : ndarray
        The x-coordinates for every pixel within the considered frame.
    y :  ndarray
        The y-coordinates for every pixel within the considered frame.
    x0 : float
        X position guess for centroid.
    y0 : float
        Y position guess for centroid.
    Returns
    -------
    ndarray 
        A 2D array of a 2D gaussian formula.
    Notes
    -----
    History:
    - Feb 22, 2023 Isaac Edelman 
        Initial implementation.
    """
    # unpack params
    amplitude, x_stddev, y_stddev = params
    # Formula for a 2D gaussian function
    return (amplitude*np.exp(-(x-x0)**2/(2*x_stddev**2) -
                             (y-y0)**2/(2*y_stddev**2)))