The second derivative above occurs so often it has its own name and notation. It is called the Laplacian of the function $V$, and is written in any of the forms \begin{eqnarray*} \triangle V = \nabla^2 V = \grad\cdot\grad V \end{eqnarray*} In rectangular coordinates, it is easy to compute \begin{eqnarray*} \triangle V = \grad\cdot\grad V = \PARTIAL{V}{x} + \PARTIAL{V}{y} + \PARTIAL{V}{z} \end{eqnarray*}