# HOW TO MAINTAIN THE GEOMETRY OF A NANOTUBE OF TiO2 IN NORMAL CONDITIONS?

Dear All
I am trying to simulate a TiO2 nanotube (rutile), for it I used NVT to 300K and Buckingham potential in the Matsui-Akaogi force field, with this I managed to relax its temperature and pressure from 1pic second, however the trajectories show that the nanotube tends to collapse as time passes, so that it will deform and losing almost its tube shape. I have tried with different radius and thickness but I get the same problem of collapse. How can I get the TiO2 nantube to maintain its geometry under normal conditions? Maybe these nanotubes can not be obtained under normal conditions and require external forces that support the structure or fix certain atoms?

# ==================initial parameters ===================================================

variable T_depart equal 300
variable dt equal 0.0005

units metal
atom_style charge
dimension 3
boundary p p p

mass 2 47.867
group Ti type 2
compute chargeTi Ti property/atom q
compute q_Ti Ti reduce ave c_chargeTi

mass 1 16.00
group Oxy type 1
compute chargeOxy Oxy property/atom q
compute q_Oxy Oxy reduce ave c_chargeOxy

velocity all create \${T_depart} 277387

#=================Potential =============================================================
pair_style buck/coul/long 2.5
pair_coeff 1 1 11783 0.234 30.220 # O -O
pair_coeff 1 2 16958 0.194 12.590 # O -Ti
pair_coeff 2 2 31120 0.154 5.250 # Ti-Ti

kspace_style ewald 1.0e-4

neighbor 0.5 bin
neigh_modify every 20 delay 0 check yes

timestep \${dt}

#================= Thermo=========================================================
thermo_style custom step temp press pe ke etotal c_q_Ti c_q_Oxy lx ly lz vol
thermo_modify flush yes
thermo 1

dump 5 all custom 5 Tubo.lammpstrj id type q x y z
dump_modify 5 sort id

#================== Minimization ========================================================
minimize 1.0e-4 1.0e-8 1000 10000

#=================== Equilibration ======================================================
thermo 1
fix 1 all nvt temp 300.0 300.0 \$(100.0*dt)

run 10000

if the force field does not maintain the geometry, then you are hitting a limit of the force field. the only way to maintain the geometry in such a case, is to not let the object change its geometry at all, e.g. by using fix rigid.

i donâ€™t know this particular parameterization, but i would be surprised, if a force field without directional components can maintain a nanotube geometry. has the force field parameterized for this purpose? or perhaps only for bulk materials? for classical modeling of oxide surfaces, i would also expect some kind of charge equilibration scheme, since the partial charges would change at all surfaces vs bulk.

please keep in mind that in classical potentials there are many approximations and assumptions and thus the potentials are not as transferable as, say, quantum chemical calculations.

axel.