First, consider the H-matrix for H 2 O using a high-level potential. Restriction of the basis was done using an excitation space up to triples. Point group symmetry was also used. The resulting H-matrix consists of two blocks (A 1 and B 2 ) of orders 161 and 125, respectively. (Even with these small matrices, the eigenvalues for the fundamentals and the bend overtone are within less than 1.0 cm -1 of "exact" results.) A histogram of the matrix elements for the A 1 block is given in Figure . As seen many elements are small; 1 192 of the 13 041 elements are less than 10 -10 hartree (0.00002 cm -1 ) in magnitude and so can be classified as negligible. Of course many more elements are also small, e.g., 7 310 have magnitude less than 10 -5 hartree (2 cm -1 ). Whether these can classified as negligible, i.e., assigned the value zero, or not requires quantitative investigation and that is done after histograms for vinyl and glycine are given.