The total number of different relative distances r_{ij} is N(N-1)/2 . In a matrix storage format, the relative distances form a strictly upper triangular matrix
\mathbf{r} \equiv \begin{pmatrix} 0 & r_{1,2} & r_{1,3} & \cdots & r_{1,N} \\ \vdots & 0 & r_{2,3} & \cdots & r_{2,N} \\ \vdots & \vdots & 0 & \ddots & \vdots \\ \vdots & \vdots & \vdots & \ddots & r_{N-1,N} \\ 0 & 0 & 0 & \cdots & 0 \end{pmatrix}.This applies to \mathbf{g} = \mathbf{g}(r_{ij}) as well.
In our algorithm we will move one particle at the time, say the kth -particle. This sampling will be seen to be particularly efficient when we are going to compute a Slater determinant.