documentclass[a4paper, 12pt, twoside,openright]{report}
usepackage{lmodern}
usepackage[upright]{fourier}

% declare `cmex` to be arbitrary scalable
DeclareFontShape{OMX}{cmex}{m}{n}{
  <-7.5> cmex7
  <7.5-8.5> cmex8
  <8.5-9.5> cmex9
  <9.5-> cmex10
}{}

SetSymbolFont{largesymbols}{normal}{OMX}{cmex}{m}{n}
SetSymbolFont{largesymbols}{bold}  {OMX}{cmex}{m}{n}

usepackage{amsmath}
usepackage{amssymb}
usepackage{bm}

begin{document}
    begin{equation}
    U_{qq} = sum_{i < j leq N_m} q_i q_j left[ sideset{}{'}sum_{bm{n}} frac{text{erfc}(alpha lvertbm{r}_{ij} + Lbm{n}rvert)}{lvertbm{r}_{ij} + Lbm{n}rvert} + frac{1}{pi L} sum_{bm{n} not= 0} frac{1}{n^2} exp(- frac{pi^2 n^2}{alpha^2 L^2} + frac{2 pi i}{L} bm{n} cdot bm{r}_{ij}) right]
    end{equation}
end{document}