  Upon chemical substitution of oxygen with fluor, LaMnAsO has been
electron-doped in experiments, resulting in samples with remarkably high
Seebeck coefficients of around -300$\mu VK^{-1}$ at room temperature and 3%
doping. Within the framework of density functional theory plus dynamical
mean-field theory we are not only able to reproduce these experimental
observations, but can also provide a thorough investigation of the underlying
mechanisms. By considering electronic correlations in the half-filled Mn-3d
shells, we trace the high Seebeck coefficient back to an asymmetry in the
spectral function, which is due to emergence of incoherent spectral weight
under doping and a strong renormalisation of the unoccupied states. This is
only possible in correlated systems and cannot be explained by DFT-based band
structure calculations.
