In principle, the choice of reference species is arbitrary, but there are some constraints. Reference species have to be chosen so that there is a species for every element. For example, we can use A=[CH 4 , H 2 O, H 2 ] as closed-shell anchor species to determine the chemical potentials. It is assumed that these reference species are available in thermodynamic reservoirs . These reservoirs are connected with the unit cell of the target molecule/adsorbate/structure, and the reference species can be drawn from the reservoirs or vanish in them without affecting the pressure or temperature of the target . The chemical potentials are then derived, assuming a thermoneutral reaction to form the element from the closed-shell reference species.