This paper describes a method for predicting which surfaces of a cryst
al can form reflection twins. The algorithm has been successfully appl
ied to six inorganic crystal systems ranging from simple salts (corund
um and rutile) to more complicated systems containing molecular ions (
aragonite, gibbsite, sodium oxalate and zircon). Our approach calculat
es the difference between the attachment energies of twinned and untwi
nned growth slices. The occurrence of planes that result in a small di
fference of energy (<1% of the intermolecular lattice energy) correlat
es with all the commonly observed twinning in these systems.