  In the low energy effective theory describing the partially quenched
extension of two light Wilson fermions, three low energy constants (LECs)
appear in terms proportional to a^2 (a being the lattice spacing). We propose
methods to separately calculate these LECs, typically called W'_6, W'_7 and
W'_8. While only one linear combination of these constants enters into physical
quantities, different combinations enter into the description of the spectral
density and eigenvalue distributions of the lattice Dirac operator and its
Hermitian counterpart. Thus it is useful to be able to determine the LECs
separately. Our methods require studying certain correlation functions for
either two or three pion scattering, which are accessible only in the partially
quenched extension of the theory.
