  We present a method for customizing the root grid of zoom-in initial
conditions used for simulations of galaxy formation. Starting from the white
noise used to seed the structures of an existing initial condition, we cut out
a smaller region of interest and use this trimmed white noise cube to create a
new root grid. This new root grid contains similar structures as the original,
but allows for a smaller box volume and different grid resolution that can be
tuned to best suit a given simulation code. To minimally disturb the zoom
region, the dark matter particles and gas cells from the original zoom region
are placed within the new root grid, with no modification other than a bulk
velocity offset to match the systemic velocity of the corresponding region in
the new root grid. We validate this method using a zoom-in initial condition
containing a Local Group analog. We run collisionless simulations using the
original and modified initial conditions, finding good agreement. The dark
matter halo masses of the two most massive galaxies at $z=0$ match the original
to within 15%. The times and masses of major mergers are reproduced well, as
are the full dark matter accretion histories. While we do not reproduce
specific satellite galaxies found in the original simulation, we obtain
qualitative agreement in the distributions of the maximum circular velocity and
the distance from the central galaxy. We also examine the runtime speedup
provided by this method for full hydrodynamic simulations with the ART code. We
find that reducing the root grid cell size improves performance, but the
increased particle and cell numbers can negate some of the gain. We test
several realizations, with our best runs achieving a speedup of nearly a factor
of two.
