The electron probability distribution function, ρ, defined as \[\rho (\mathbf{r}) = n\ \int \Psi ^{\text{*}}\left[\mathbf{r}(1),\mathbf{r}(2)\,\text{...}\,\mathbf{r}(n)\right]\ \Psi \left[\mathbf{r}(1),\mathbf{r}(2)\,\text{...}\,\mathbf{r}(n)\right]\text{d}\mathbf{r}(2)\,\text{...}\,\text{d}\mathbf{r}(n)\] where Ψ is an electronic wave-function and integration is made over the coordinates of all but the first electron of n. The physical interpretation of the electron density function is that ρ d r gives the probability of finding an electron in a volume element dr, i.e., electron density in this volume.