Suppose we have a 3d lattice of unit cells. In each unit cell we have a number of Gaussians $G(\boldsymbol{r}-\boldsymbol{R}_m-\boldsymbol{R}_n)$ sitting at a specific site $m$ in the unit cell with the lattice vector $ \boldsymbol{R}_n$. Now we want to calculate the Fourier transform of the sum of all these Gaussians. Suppose the sum goes to infinity, the sum should yield a 100% periodic function, so I thought this should not be a problem:
$\mathcal{F}\left(\sum\limits_{n=0}^\infty\underbrace{\sum\limits_{m=0}^{N_\mathrm{sites}} G(\boldsymbol{r}-\boldsymbol{R}_m-\boldsymbol{R}_n)}_{f(\boldsymbol{r}-\boldsymbol{R}_n)}\right)$
I thought about using the Poisson summation formula, but this gives me back only again a sum over Gaussians. I thought that maybe in reciprocal space we can get rid of the sum over unit cells by introducing a properly periodic function...
I add to this question that actually $\int_{\Omega} d\boldsymbol{r} f(\boldsymbol{r}) = 0$ for $\Omega$ being the unit cell volume. This is required to make the sum converge.