There are significant differences between the atomic masses in NIST PML database and pyqchem/constants.py:
Z= 3, nist = 7.0160034366 , pyqchem = 6.015122795
Z= 5, nist = 11.00930536 , pyqchem = 10.012937
Z=18, nist = 39.9623831237 , pyqchem = 35.967545106
Z=22, nist = 47.94794198 , pyqchem = 45.9526316
Z=23, nist = 50.94395704 , pyqchem = 49.9471585
Z=24, nist = 51.94050623 , pyqchem = 49.9460442
Z=26, nist = 55.93493633 , pyqchem = 53.9396105
Z=32, nist = 73.921177761 , pyqchem = 69.9242474
Z=34, nist = 79.9165218 , pyqchem = 73.9224764
Z=36, nist = 83.9114977282 , pyqchem = 77.9203648
Z=38, nist = 87.9056125 , pyqchem = 83.913425
Z=42, nist = 97.90540482 , pyqchem = 91.906811
Z=43, nist = 97.9072124 , pyqchem = 96.906365
Z=44, nist = 101.9043441 , pyqchem = 95.907598
Z=46, nist = 105.9034804 , pyqchem = 101.905609
Z=48, nist = 113.90336509 , pyqchem = 105.906459
Z=49, nist = 114.903878776 , pyqchem = 112.904058
Z=50, nist = 119.90220163 , pyqchem = 111.904818
Z=52, nist = 129.906222748 , pyqchem = 119.90402
Z=54, nist = 131.9041550856 , pyqchem = 123.905893
Z=56, nist = 137.905247 , pyqchem = 129.9063208
Z=57, nist = 138.9063563 , pyqchem = 137.907112
Z=58, nist = 139.9054431 , pyqchem = 135.907172
Z=62, nist = 151.9197397 , pyqchem = 143.911999
Z=63, nist = 152.921238 , pyqchem = 150.9198502
Z=64, nist = 157.9241123 , pyqchem = 151.919791
Z=66, nist = 163.9291819 , pyqchem = 155.924283
Z=68, nist = 165.9302995 , pyqchem = 161.928778
Z=70, nist = 173.9388664 , pyqchem = 167.933897
Z=72, nist = 179.946557 , pyqchem = 175.9414086
Z=73, nist = 180.9479958 , pyqchem = 179.9474648
Z=74, nist = 183.95093092 , pyqchem = 179.946704
Z=75, nist = 186.9557501 , pyqchem = 184.952955
Z=76, nist = 191.961477 , pyqchem = 183.9524891
Z=77, nist = 192.9629216 , pyqchem = 190.960594
Z=78, nist = 194.9647917 , pyqchem = 189.959932
Z=80, nist = 201.9706434 , pyqchem = 195.965833
Z=81, nist = 204.9744278 , pyqchem = 202.9723442
Z=82, nist = 207.9766525 , pyqchem = 203.9730436
Z=86, nist = 222.0175782 , pyqchem = 210.990601
Z=88, nist = 226.0254103 , pyqchem = 223.0185022
Z=90, nist = 232.0380558 , pyqchem = 230.0331338
Z=92, nist = 238.0507884 , pyqchem = 233.0396352
Z=93, nist = 237.0481736 , pyqchem = 236.04657
Z=94, nist = 244.0642053 , pyqchem = 238.0495599
Even after discounting Z=43, 61, 84--89, 93, 94 (for which the NIST database does not give isotopic composition but the most stable isotope) the result suggests maybe the lightest isotope was used instead of the most common, contradicting the comment on line 216 # Relative atomic mass per most common isotope?
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