  Meaningful predictions for electric quadrupole (E2) observables from ab
initio nuclear theory are necessary, if the ab initio description of collective
correlations is to be confronted with experiment, as well as to provide
predictive power for unknown E2 observables. However, converged results for E2
observables are notoriously challenging to obtain in ab initio no-core
configuration interaction (NCCI) approaches. Matrix elements of the E2 operator
are sensitive to the large-distance tails of the nuclear wave function, which
converge slowly in an oscillator basis expansion. Similar convergence
challenges beset ab initio prediction of the nuclear charge radius. We
demonstrate that the convergence patterns of the E2 and radius observables are
strongly correlated, and that meaningful predictions for the absolute scale of
E2 observables may be made by calibrating to the experimentally-known
ground-state charge radius. We illustrate by providing robust ab initio
predictions for several E2 transition strengths and quadrupole moments in
p-shell nuclei, in cases where experimental results are available for
comparison.
