Nonminimally coupled scalar field

A nonminimal scalar coupling includes an interaction between the field and spacetime curvature, such as $-\xi R{\Phi }^2$ in the Lagrangian density. The Euler-Lagrange field equation for $\mathcal{L}=-\frac{1}{2}(\nabla \Phi {)}^2-\xi R{\Phi }^2$ is $\square \Phi -2\xi R\Phi =0$. The coefficient is convention dependent: an action written with $-\frac{1}{2}\xi R{\Phi }^2$ has $\xi$ instead of $2\xi$.