The average window error for the frequencies is lowest for 500 snapshots with 0.2 cm -1 and gradually increases up to 1.0 cm -1 for 25 snapshots, while the standard deviation increases from 0.0 to 0.3 cm -1 . Additionally, the maximum error is below 1.0 cm -1 for 500, 250, and 125 snapshots and only increases to 2.5 cm -1 for 25 snapshots. This indicates that the error in the frequencies due to insufficient sampling can be kept quite low with as few as 25 snapshots, i.e., on the order of the basis-set incompleteness error. Similarly, the absolute and relative IR and Raman intensity errors increase gradually when decreasing the number of snapshots from 500 to 25. The relative IR intensity errors range from an average error of 1.4 % and a maximum error of 2.6 % to an average error of 6.6 % and a maximum error of 12.2 %. In comparison, the relative Raman intensity errors range from an average error of 1.1 % and a maximum error of 2.0 % to an average error of 3.8 % and a maximum error of 9.1 %. To ensure a maximum relative error below 5 % for the intensities and the maximum frequency error below 1.0 cm -1 , we chose 250 snapshots with a 2.0 ps step size to be appropriate for our calculations in Section 5.