The zero fret

The zero fret is essential to give the Smile frets (TM) an exact intonation. 
It is illogical to use a suspending nut that is made out of a material, different from the fret wire because the open strings will sound different compared to the depressed strings.
We solve that problem by using the zero fret. The zero fret is made out of the same material as the other frets (hard metal). It balances and equilibrates the sound of the open string chords.
In other guitars, the nut has a double function. 

It keeps the strings at distance from each other and it levels them (height adjustment).

A kritz has a zero fret that serves as a perfect height adjustment. 
The Kritzwood nut keeps the string at an equal distance from each other. 
In this way, tensions are divided in 2 groups. 
Together with a straight string pull, it is the most simple and most effective way to have a stable and accurate open tuning system.