  We assess the predictive power of alchemical perturbations for estimating
fundamental properties in ionic crystals. Using density functional theory we
have calculated formation energies, lattice constants, and bulk moduli for all
sixteen iso-valence-electronic combinations of pure pristine alkali halides
involving elements $A \in \{$Na, K, Rb, Cs$\}$ and $X \in \{$F, Cl, Br, I$\}$.
For rock salt, zincblende and cesium chloride symmetry, alchemical
Hellmann-Feynman derivatives, evaluated along lattice scans of sixteen
reference crystals, have been obtained for all respective 16$\times$15
combinations of reference and predicted target crystals. Mean absolute errors
(MAE) are on par with density functional theory level of accuracy for energies
and bulk modulus. Predicted lattice constants are less accurate. NaCl is the
best reference salt for alchemical estimates of relative energies (MAE $<$ 40
meV/atom) while alkali fluorides are the worst. By contrast, lattice constants
are predicted best using NaF as a reference salt (MAE $<$ 0.5{\AA}), yielding
only semi-quantitative accuracy. The best reference salt for the prediction of
bulk moduli is CsCl (MAE $<$ 0.4$\times$10$^{11}$ dynes/cm$^2$). Alchemical
derivatives can also be used to predict competing rock salt and cesium chloride
phases in binary and ternary solid mixtures with CsCl. Alchemical predictions
based on dispersion corrected density functional theory with pure RbI as a
reference salt reproduce reasonably well the reversal of the rock salt/cesium
chloride stability trend for binary $(AX)_{1-x}$CsCl$_x$ as well as for ternary
$(AX)_{0.5-0.5x}(BY)_{0.5-0.5x}$CsCl$_x$ mixtures.
