  The reverse perturbation method [Phys. Rev. E 59, 4894 (1999)] for shearing
simple liquids and measuring their viscosity is extended to the Vicsek-model
(VM) of active particles [Phys. Rev. Lett. 75, 1226 (1995)] and its metric-free
version. The sheared systems exhibit a phenomenon that is similar to the skin
effect of an alternating electric current: momentum that is fed into the
boundaries of a layer decays mostly exponentially towards the center of the
layer. It is shown how two transport coefficients, i.e. the shear viscosity
$\nu$ and the momentum amplification coefficient $\lambda$, can be obtained by
fitting this decay with an analytical solution of the hydrodynamic equations
for the VM. The viscosity of the VM consists of two parts, a kinetic and a
collisional contribution. While analytical predictions already exist for the
former, a novel expression for the collisional part is derived by an
Enskog-like kinetic theory. To verify the predictions for the transport
coefficients, Green-Kubo relations were evaluated and transverse current
correlations were measured in independent simulations. Not too far to the
transition to collective motion, we find excellent agreement between the
different measurements of the transport coefficients. However, the measured
values of $\nu$ and $1-\lambda$ are always slightly higher than the mean-field
predictions, even at large mean free paths and at state points quite far from
the threshold to collective motion, that is, far in the disordered phase. These
findings seem to indicate that the mean-field assumption of molecular chaos is
much less reliable in systems with velocity-alignment rules such as the VM,
compared to models obeying detailed balance such as Multi-Particle Collision
Dynamics.
