  Quantum motion of atoms known as zero-point vibrations is recognized to be
important at low temperatures in condensed matter systems comprised of light
atoms or ions, affecting such properties and behaviors as proton-transfer
reactions, vibrational spectra of water and ice, and mechanical properties of
low temperature helium. Recently, quantum motion of atoms was proposed to
explain a long-standing discrepancy between theoretically computed and
experimentally measured low-temperature resistance (Peierls stress) to
dislocation motion in iron and possibly other metals with high atomic masses.
Here we report the first direct simulations of quantum motion of screw
dislocations in iron within the exact formalism of Ring-Polymer Molecular
Dynamics (RPMD) that rigorously accounts for quantum effects on the statistics
of condensed-phase systems. Our quantum RPMD simulations predict only a modest
($\approx\!13\%$) reduction in the Peierls stress in iron compared to its fully
classical prediction. Our simulations confirm that reduction in the Peierls
stress solely due to the zero-point energy is close to $50\%$ predicted
earlier, but its effect is substantially offset by an increase in the effective
atom size with decreasing temperature, an effect known as quantum dispersion.
Thus, quantum motion of atoms does not resolve the notorious discrepancy
between theoretical and experimental values of the Peierls stress in iron.
