Self-adjointness of the perturbed wave operator on $L^2({\bf R^n}), n\geq 2$

We give classes of unbounded real-valued $V$ for which $\square+V$ is self-adjoint on $\mathcal{D}(\square)\subset L^2({\bf R^n})$, $n\geq 2$, where $\square$ is the wave operator defined on ${\bf R^n}$.