def G_matrix(q, r, contrast, FFfxn, Volfxn, args=()):
    """Calculates the response matrix :math:`G(Q,r)` 
    
    :param float q: :math:`Q`
    :param float r: :math:`r`
    :param float contrast: :math:`|\\Delta\\rho|^2`, the scattering contrast
    :param function FFfxn: form factor function FF(q,r,args)
    :param function Volfxn: volume function Vol(r,args)
    :returns float: G(Q,r)
    """
    FF = FFfxn(q, r, args)
    Vol = Volfxn(r, args)
    return 0.0001 * (contrast * Vol * FF ** 2).T
