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I use vasp to simulation carbon nanotube ,the import text I do relaxtion as follow,I want to known they are all right?
SYSTEM = Carbon nanotubes in a box
ENCUT = 500
ISTART = 0;ICHARG = 2
ISMEAR = 0;SIGMA = 0.1
NSW = 100;IBRION = 2
ISIF = 3
POTIM = 0.1
EDIEF = 0.1E-04;EDIFFG = -0.3E-2
PREC = Accurate
LREAL = Auto

Prevent output of big files:
LCHARG = .FALSE.
LWAVE = .FALSE.



Monkhorst Pack
0
Monkhorst Pack
1 1 30
0 0 0



Carbon nanotube(2,2) in a box
11.86 ! universal scaling parameters
1.0000000 0.0000000 0.0000000 ! lattice vector a(1)
0.0000000 1.0000000 0.0000000 ! lattice vector a(2)
0.0000000 0.0000000 0.2074000 ! lattice vector a(3)
16 ! number of atoms
Direct ! positions in cartesian coordinates
0.739460 0.480341 0.000000
0.708803 0.594759 0.000000
0.672437 0.642145 0.500244
0.569857 0.701369 0.500244
0.510632 0.709169 0.000000
0.396214 0.678511 0.000000
0.348828 0.642145 0.500244
0.289604 0.539565 0.500244
0.281804 0.480341 0.000000
0.312462 0.365922 0.000000


PAW C 31May2000
4.00000000000000000

there is no use to relax the cell shape (ISIF=3) for a nanotube, I think. please choose
ISIF=2
in INCAR and make use ot the 'selective dynamics' feature to be set in POSCAR (http://cms.mpi.univie.ac.at/vasp/vasp/node63.html)
( a line
selective dynamics
has to be inserted before the
Direct
line, then specify a triplet of T or F after each atom's position in order to specify whether the atom is allowed to relax along the respective co-ordinate:
eg: F F T means that the atom relaxes along the z-direction only, the x and y co-ordinates are kept fixed.

hello my dear firend
i also use the vasp to somulation carbon nanotube
i am cosidering the ban structure an DOS of achiral carbon nanotube.
please tell me why the vector lattice is 1.0 0.0 0.0
0.0 1.0 0.0
0.0 0.0 .27
in the other han how you derive them
i recive the other result
the best regards
thanks