To proceed we determine the quantitative relationship between the magnitude of matrix elements that are be set to zero, based on a threshold, and the resulting mean absolute error (MAE) of all the eigenvalues for each H-matrix. (The number is the same as the order of the matrix and this is given for convenience in the second column of Table .) This relationship is shown in Figure for five H-matrices. The two additional ones are for 3-mode and 4-mode vinyl. The latter one is the exception mention above that was not obtained with MULTIMODE. It was obtained used saddle-point normal modes, following earlier work. As seen, and as expected, the fraction of elements set to zero and the MAE of all eigenvalues increases with increasing threshold. For the threshold of 10 -5 hartree (2.2 cm -1 ) the MAE is at most 1 cm -1 . Also at that threshold the percentage of zeroed elements increases from about 25 to 90+ percent. For glycine with a percentage just below 100 %, the histogram includes numerous elements that are set to zero in MULTMODE without calculating them. Glycine is the only example where this occurs and as noted earlier we provide the explanation below, where we examine the n-mode representation of the potential.