In the case of tensors the colon notation of array slices as in python's Numpy is used. So the i-th column vector of the matrix X may be written as X :,i and the j-th row vector may be written as X j,: . Another important convention is the layout of coordinates. The row wise alignment of XYZ files makes sense for these CSV like files. But in the following section it is mathematically advantageous to use a column wise alignment of coordinates. This implies that the cartesian positions and Z-matrix entries of a molecule with n atoms are an element of R 3,n .