The expression of Poisson’s equation (2.1) on page 4 is incorrect. The equation should read:
(2.1)
$$\begin{eqnarray}\frac{\unicode[STIX]{x2202}^{2}\unicode[STIX]{x1D719}}{\unicode[STIX]{x2202}x^{2}}+\mathop{\sum }_{j=1}^{N}Z_{j}n_{j}=0.\end{eqnarray}$$
$$\begin{eqnarray}\frac{\unicode[STIX]{x2202}^{2}\unicode[STIX]{x1D719}}{\unicode[STIX]{x2202}x^{2}}+\mathop{\sum }_{j=1}^{N}Z_{j}n_{j}=0.\end{eqnarray}$$
