  We calculate the equation of state of neutron matter at zero temperature by
means of the auxiliary field diffusion Monte Carlo method (AFDMC) combined with
a fixed-phase approximation. The calculation of the energy is carried out by
simulating up to 114 neutrons in a periodic box. Special attention was made to
reduce finite size effects at the energy evaluation by adding to the
interaction the effect due to the truncation of the simulation box, and by
performing several simulations using different number of neutrons. The finite
size effects due to the kinetic energy were also checked by employing the
twist--averaged boundary conditions. We considered a realistic nuclear
Hamiltonian containing modern two-- and three--body interactions of the Argonne
and Urbana family. The equation of state can be used to compare and to
calibrate other many-body calculations and to predict properties of neutron
stars.
