The sources are:

Symbol
$a_{\mathrm{VdW}}$
Unit
bar.L²/mol²
Used in
$p_{\mathrm{VdW}}$
Choices
GasFormulaa
Air-1.358
N2N21.37
SF6SF67.857
CO2CO23.658
COCO1.472
O2O21.382
H2H20.2453
NH3NH34.225
SO2SO26.865
HeHe0.0346
ArAr1.355
KrKr2.325
XeXe4.192