  The effective interaction between a nucleon and a nucleus is one of the most
important ingredients for reaction theories. Theoretical formulations were
introduced early by Feshbach and Watson, and efforts of deriving and computing
those `optical potentials' in a microscopic fashion have a long tradition.
However, only recently the leading order term in the Watson multiple scattering
approach could be calculated fully {\it ab initio}, meaning that the same
nucleon-nucleon (NN) interaction enters both the structure as well as the
reaction pieces on equal footing. This allows the uncertainties from the
underlying chiral effective NN interaction to be systematically explored in
nucleon-nucleus elastic scattering observables.
  In this contribution the main ingredients for arriving at the {\it ab initio}
leading order of the effective nucleon-nucleus interaction in the Watson
approach will be reviewed. Concentrating on one specific chiral NN interaction
from the LENPIC collaboration and light nuclei with a 0$^+$ ground state, the
leading order nucleon-nucleus interaction is calculated using up to the third
chiral order (N2LO) in the nucleon-nucleon potential, and elastic scattering
observables are extracted. Then pointwise as well as correlated uncertainty
quantification is used for the estimation of the chiral truncation error.
Elastic scattering observables for $^4$He, $^{12}$C, and $^{16}$O for between
65 and 200 MeV projectile energy will be analyzed.
