When using a stationary solver and when a chemical mechanism contains species made up of different elements, the default mass constraint (the sum of all mass fractions equals one) might not be good enough to ensure mass conservation over all elements. The Species Group feature can be used to ensure that the total elemental mass in the system is conserved. Consider a simplified Ar/O2 plasma chemistry with an initial 70/30 ratio by mole fraction. For a closed system (which is currently the only type of system currently supported in the Plasma, Time Periodic interface), the sum of the mass fractions of oxygen species should equal the initial value, and the sum of the mass fractions of argon species should equal their initial values. If the following species are considered in the model: Ar, Ars, Ar_1p, O2, O, O2_1p, then ground state argon should be picked as the From mass constraint species, and a Species Group feature should be added with the following table:

Note that the first species in the table matters. The first species in the table is used to enforce mass conservation for this group of species, meaning that the total number of oxygen atoms in the system is conserved. The first species should correspond to a ground state species present in excess within this group. If additional species N2, N2A, N2_1p were added to the model, an additional Species Group feature would be necessary: