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Dear Ibrahim,

I wouldn't use MD simulations to do this, I would simply strain the cell and perform a geometry optimisation with that fixed cell but varying the ionic positions, i.e.

  fix_all_cell : true

  fix_all_ions : false

in the .cell file, and tell CASTEP to also compute the stress, i.e.

  calculate_stress : true

in the .param file. This will give you the stress for that strain; repeat with different strains, and you have the stress-strain curve. 

An alternative approach is to do a variable-cell geometry optimisation under an applied stress, e.g.

  fix_all_cell : false

  fix_all_ions : false

  %block external_pressure

     GPa

     0.0    0.0    0.0

               0.0    0.0

                       10.0 

  %endblock external_pressure

in the .cell file to apply a 10 GPa stress along z (the strange-looking input structure is because the tensor has to be symmetric, so we only let users set the independent elements).

If you do this, you probably want to constrain the lattice in the plane perpendicular to the nanotube and only vary in the nanotube direction. If the nanotube is along the c-axis, then you'd put:

  %block cell_constraints

  0 0 1

  0 0 0

  %endblock cell_constraints

in the .cell file to fix a and b, vary c, and fix alpha, beta and gamma (the cell angles).

Hope that helps.

Phil Hasnip

(CASTEP developer)

Dear Philip,

Which module do you suggest for nanotube calculations DMol3 or CASTEP?

As a CASTEP developer, I'd always recommend CASTEP!

More seriously, I have always used CASTEP for nanotubes but I've never actually run DMol3 so I can't honestly say which is better. If you want an isolated, finite-length nanotube then DMol3 might be the best option; for a large cluster of very long nanotubes you'd probably want to model them as 3D periodic so CASTEP would be best. In between we have short nanotubes, which would require the simulation to be aperiodic along the tube, and/or an isolated nanotube which would require it to be aperiodic in the directions perpendicular to the tube... As I said, I'd always use CASTEP but you should probably test both (at an equivalent accuracy level). For extremely large simulations you might also consider trying ONETEP (a linear-scaling DFT code).

Sorry I can't be more help,

Phil

Philip James Hasnip SIR

I am working to find piezoelectric properties .but unable to find the piezoelectric tensor through CASTEP .need your valuable suggestions.

Muhammad Dawood we don't currently calculate the piezoelectric tensor as a task within CASTEP. If you don't mind a little programming, you could adapt the elastic constants scripts to do a finite-difference of the Berry phase polarisation (you'd have to turn that on in the CASTEP param files). Are you wanting the clamped-ion or free-ion piezoelectric tensor?

I think BIOVIA might have a Pipeline Pilot protocol to do it, as well.