  A key component of the phase diagram of many iron-based superconductors and
electron-doped cuprates is believed to be a quantum critical point (QCP),
delineating the onset of antiferromagnetic spin-density wave order in a
quasi-two-dimensional metal. The universality class of this QCP is believed to
play a fundamental role in the description of the proximate non-Fermi liquid
and superconducting phases. A minimal model for this transition is the
$\mathrm{O}(3)$ spin-fermion model. Despite many efforts, a definitive
characterization of its universal properties is still lacking. Here, we
numerically study the $\mathrm{O}(3)$ spin-fermion model and extract the
scaling exponents and functional form of the static and zero-momentum dynamical
spin susceptibility. We do this using a Hybrid Monte Carlo (HMC) algorithm with
a novel auto-tuning procedure, which allows us to study unprecedentedly large
systems of $80 \times 80$ sites. We find a strong violation of the Hertz-Millis
form, contrary to all previous results. Furthermore, the form that we do
observe provides good evidence that the universal scaling is actually governed
by the analytically tractable fixed point discovered near perfect ``hot-spot'"
nesting, even for a larger nesting window. Our predictions can be directly
tested with neutron scattering. Additionally, the HMC method we introduce is
generic and can be used to study other fermionic models of quantum criticality,
where there is a strong need to simulate large systems.
