In section II, we first discuss the underlying physics and mathematics of both BW perturbation theory as well as BW-s2(α) (sections II A -II G). Next, we explore the structure, properties and behaviour of general one-body regularisation operators in BW2 (section II E) and, on the basis of these results, present an algorithm for constructing arbitrary sizeconsistent BW2 variants (sections II G -II H). Lastly, we provide physical interpretation for the constructed Hamiltonian (section III A) and discuss optimal values for parameters by considering small model systems (section III B).