DerivedQuantities can depend on other derived quantities, but currently Observables must depend on S and the field variables.
However, some primary observables are simple manipulations of other primary observables. For example, the spin susceptibility is really just the sum of the Spin_Spin correlation function with no disconnected piece. It would be nice to be able to measure the susceptibility as a primary observable without recalculating the whole expensive mess in the Spin_Spin correlation function.
Currently the susceptibility is a DerivedQuantity but that's purely because it is most easily written as a function of a primary observable (ie. sum).
DerivedQuantities can depend on other derived quantities, but currently Observables must depend on S and the field variables.
However, some primary observables are simple manipulations of other primary observables. For example, the spin susceptibility is really just the sum of the Spin_Spin correlation function with no disconnected piece. It would be nice to be able to measure the susceptibility as a primary observable without recalculating the whole expensive mess in the Spin_Spin correlation function.
Currently the susceptibility is a DerivedQuantity but that's purely because it is most easily written as a function of a primary observable (ie. sum).