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A very simple python script to generate nanobue from nanosheet formed by any crystal. 
The user only needs to provide a strcture file in the format of VASP/POSCAR and prepare
an supercell input file as illustrated in the following. 

We first create a new supercell.in with contents as following (Chrysotile.vasp is crystal structure in primitive unit cell) : 
Chrysotile.vasp
1  0  0 
1  2  0 
0  0  1
1
0

Save the file and run 
>>> python construct_nanotube.py
>>> cp nanosheet.vasp Chrysotile_rect.vasp 

Now you have a new crytal structure which a1_new = a1_old, a2_new = 1*a1_old + 2*a2_old, satisfying a2_new PENPENDICULAR to a1_new.
If you want other structure, please modify the 2nd to 4th line as you wish
a11 a12 a13 
a21 a22 a23
a31 a32 a33 
such that a1_new = a11*a1_old + a12*a2_old + a13*a3_old; a2_new = a21*a1_old + a22*a2_old + a23*a3_old, etc. 
Please make sure that a1_new should be perpendicular to a2_new. and a3_new is permendicular to a1_new and a2_new. 

Now we use supercell.in with contents as following 
Chrysotile_rect.vasp
24  0    0 
0   3   0 
0   0   1
1
0

Save the file and run 
>>> python construct_nanotube.py

This will generate a nanotube which rolls along a1_new direction with the tube central axis // to the a2_new direction. 

The 5th line can be 1 or -1, settting whether the top most atoms in z are rolled inside or outside.
The 6th line can be 0 to |c_new|, which sets how much we shift the atoms in z before we roll a nanosheet into a tube. 


Copyright: 
Changming Yue
yuechangming8@gmail.com

Ref: cif2tube, https://doi.org/10.1016/j.jtice.2016.08.030