hello
about VLAB I'll ask to the developer...
As for the MULTIWALL nanotubes.. there is another procedure you can follow, which use the keyword MULTIWALL. These are the steps, using as a test system a graphene sheet:
1) build a two-wall nanotube ---> see 2wall.jpg

Title
SLAB
77
2.47
1
6 0.33333333 0.6666666667 0.000000
NANOMULTI
2
WALL
1
ROLLINGV
6 6
WALL
2
ROLLINGV
12 12
NANOJMOL
ENDWALL
[Basis Set]

2. use the MULTIWALL.DAT file as a fort.34 and restart with the EXTERNAL keyword substituting one of the two wall with different atom(s). In this case, the inner wall becomes a nitrogen wall, see ---> ext2wall.jpg

Title
EXTERNAL
ATOMSUBS
24
1 7
2 7
3 7
4 7
5 7
6 7
7 7
8 7
9 7
10 7
11 7
12 7
13 7
14 7
15 7
16 7
17 7
18 7
19 7
20 7
21 7
22 7
23 7
24 7
END
[Basis Set]

the ZnO@GaN were done exactly in this way... if you roll up a two-layers slab you end with a double wall nanotube

goodmorning,

ok... now I understand better. This is an example. I chose the space group 73 because the 78 has a mirror plane... I don't know which kind of lattice do you want to simulate ...neither if the two lattice (BeO and MgO) are commensurable ...but the idea is to build a two-layers slab, one layer of MgO and the other layer of BeO and then roll the slab
The lattice parameter is chosen randomly ...

Title
SLAB
73
3.47
4
8 0.5 0.0 0.000000
4  0.0 0.0 0.0000
12 0.5 0.0 2.200000
8  0.0 0.0 2.2000
SWCNT
10 10
TESTGEOM
END
END

Hello,
thank you for your question. Could you be more precise? I don't understand exactly what you mean but this is the input file for a double wall nanotube created starting from a graphene layer:

title
SLAB
77 
2.47
1
6 0.33333333 0.6666666667 0.000000
NANOMULTI
2
WALL
1
ROLLINGV
6  6
WALL
2
ROLLINGV
12    12
NANOJMOL
ENDWALL
END
END

you can have a look at the Tutorial, on the Crystal web page:
https://tutorials.crystalsolutions.eu/tutorial.html?td=mwnanotube&tf=MW_tutorial

I hope it helps...