  The amount of vapor in the impact-generated protolunar disk carries
implications for the dynamics, devolatilization, and moderately volatile
element (MVE) isotope fractionation during lunar formation. The equation of
state (EoS) used in simulations of the giant impact is required to calculate
the vapor mass fraction (VMF) of the modeled protolunar disk. Recently, a new
version of M-ANEOS ("Stewart M-ANEOS") was released with an improved treatment
of heat capacity and expanded experimental Hugoniot. Here, we compare this new
M-ANEOS version with a previous version ("N-SPH M-ANEOS") and assess the
resulting differences in smoothed particle hydrodynamics (SPH) simulations. We
find that Stewart M-ANEOS results in cooler disks with smaller values of VMF
and results in differences in disk mass that are dependent on the initial
impact angle. We also assess the implications of the minimum "cutoff" density
($\rho_{c}$), similar to a maximum smoothing length, that is set as a
fast-computing alternative to an iteratively calculated smoothing length. We
find that the low particle resolution of the disk typically results in $>40\%$
of disk particles falling to $\rho_c$, influencing the dynamical evolution and
VMF of the disk. Our results show that choice of EoS, $\rho_{c}$, and particle
resolution can cause the VMF and disk mass to vary by tens of percent.
Moreover, small values of $\rho_{c}$ produce disks that are prone to numerical
instability and artificial shocks. We recommend that future giant impact SPH
studies review smoothing methods and ensure the thermodynamic stability of the
disk over simulated time.
