Determining fractional coordinates in POSC

[chatfat]

So, this is probably a rather basic question but after searching Google for a good hour, I still haven't quite been able to figure it out. I was going over the VASP workshop slides and came across the POSCAR file for the Ni- FCC (111) structure.

Ni - (111)
3.53
.70710678 .0000000 .000000
-0.35355339 0.6123724 .000000
.000000 .000000 5.196152
5
selective dynamics
direct
.00000000 .00000000 .00000000 F F F
.33333333 .66666667 .11111111 F F F
.66666667 .33333333 .22222222 F F F
.00000000 .00000000 .33333333 T T T
.33333333 .66666667 .44444444 T T T

I'm not able to figure out where the values for the lattice vectors and the fractional coordinates come from. How exactly are they calculated/determined and how does one know which layers to relax? Any help or a nudge in the right direction would be greatly appreciated.

[chatfat]

So, here's an attempt to narrow down the question. I understand where the x and y positions for the fractional coordinates come from and I understand why they are repeating (ABC structure). What I'm not sure is how the z position is determined and which layers are relaxed.

In addition, I understand bits and pieces of the lattice vectors but am not sure about certain values (like the negative sign in -0.354 and the 5.196 in the last vector).

If someone could explain the above, that would be great. Thanks!

[admin]

Having look at numbers is not very useful. Multiply the cell in x and y dimension and display the structure using some graphical software. This will give answer to your questions.