ChPT loops for the lattice: pion mass and decay constant, HVP at finite volume and $n\bar{n}$-oscillations

I present higher loop order results for several calculations in Chiral perturbation Theory. 1) Two-loop results at finite volume for hadronic vacuum polarization. 2) A three-loop calculation of the pion mass and decay constant in two-flavour ChPT. For the pion mass all needed auxiliary parameters can be determined from lattice calculations of $\pi \pi$-scattering. 3) Chiral corrections to neutron-anti-neutron oscillations.