  The phase diagram of natural neon has been calculated for temperatures in the
range 17-50 K and pressures between 0.01 and 2000 bar. The phase coexistence
between solid, liquid, and gas phases has been determined by the calculation of
the separate free energy of each phase as a function of temperature. Thus, for
a given pressure, the coexistence temperature was obtained by the condition of
equal free energy of coexisting phases. The free energy was calculated by using
non-equilibrium techniques such as adiabatic switching and reversible scaling.
The phase diagram obtained by classical Monte Carlo simulations has been
compared to that obtained by quantum path-integral simulations. Quantum effects
related to the finite mass of neon cause that coexistence lines are shifted
towards lower temperatures when compared to the classical limit. The shift
found in the triple point amounts to 1.5 K, i.e., about 6 % of the triple-point
temperature. The triple-point isotope effect has been determined for 20Ne,
21Ne, 22Ne, and natural neon. The simulation data show satisfactory agreement
to previous experimental results, that report a shift of about 0.15 K between
triple-point temperatures of 20Ne and 22Ne. The vapor-pressure isotope effect
has been calculated for both solid and liquid phases at triple-point
conditions. The quantum simulations predict that this isotope effect is larger
in the solid than in the liquid phase, and the calculated values show nearly
quantitative agreement to available experimental data.
