  We test and improve the numerical schemes in our smoothed particle
hydrodynamics (SPH) code for cosmological simulations, including the
pressure-entropy formulation (PESPH), a time-dependent artificial viscosity, a
refined timestep criterion, and metal-line cooling that accounts for
photoionisation in the presence of a recently refined Haardt \& Madau (2012)
model of the ionising background. The PESPH algorithm effectively removes the
artificial surface tension present in the traditional SPH formulation, and in
our test simulations it produces better qualitative agreement with mesh-code
results for Kelvin-Helmholtz instability and cold cloud disruption. Using a set
of cosmological simulations, we examine many of the quantities we have studied
in previous work. Results for galaxy stellar and HI mass functions, star
formation histories, galaxy scaling relations, and statistics of the Ly$\alpha$
forest are robust to the changes in numerics and microphysics. As in our
previous simulations, cold gas accretion dominates the growth of high-redshift
galaxies and of low mass galaxies at low redshift, and recycling of winds
dominates the growth of massive galaxies at low redshift. However, the PESPH
simulation removes spurious cold clumps seen in our earlier simulations, and
the accretion rate of hot gas increases by up to an order of magnitude at some
redshifts. The new numerical model also influences the distribution of metals
among gas phases, leading to considerable differences in the statistics of some
metal absorption lines, most notably NeVIII.
