[Pw_forum] Setting parameters about carbon nanotube/metal system

[chengyu yang]

Dear All,
        I am now working on a project about carbon nanotube interacting with
metal atoms. And I've got some problems:
        1. I forgot how I defined the celldm parameters.
        Generally, I use a script for a (6,6) carbon nanotube like this:

&SYSTEM
                       ibrav = 6,celldm(1) =
21.764541128,celldm(3)=0.213765379
                         nat = 27,ntyp = 2,ecutwfc = 75 ,
                     occupations='smearing', smearing='gaussian',
degauss=0.02
 /
&ELECTRONS
                    conv_thr = 1.0d-8 ,
                 mixing_beta = 0.7 ,
 /
ATOMIC_SPECIES
    C   12.01100  C.pz-vbc.UPF
    Cu  63.55     Cu.pz-d-rrkjus.UPF

The celldm(1) and celldm(3), I don't quite understand their meaning and how
they came out. Although I could calculate the lattice parameters for carbon
nanotube, like the diameter d and the translation parameter t, I still
couldn't make connections between these parameters and the values I got
here. I forgot how I got them, and now I need to calculate a (8,8) cnt,
while I need a larger space between the carbon nanotubes, cause they are now
too crowded using the same structure data. I don't know how could I change
them.

2. I don't know what kind of pseudopotential I should choose, there are
several potentials for each element, while I am just looking for a general
calculation.
3. I keep the other parameters like the ecutwfc, the conv-thr, and the
mixing-beta the same for different metal elements, does that do harm to my
calculations? I am not quite sure.

Well, thank you for your time. I know my questions are somehow silly, but
hope one could teach me.

Thanks.

Regards.

Chengyu Yang
MMAE department, University of Central Florida
USA
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