[DFTB-Plus-User] Slow Convergence problem

[Jan M. Knaup]

Dear Bikash,

50 fs is an extremely short simulation time, especially to get a barostat
to converge, especially at 10 K where the low temperature naturally leads
to very slow atomic movment. The first question to ask would be, why do you
use 0.1 fs time steps? For hydrocarbons 0.5 - 1.0 fs are usually sufficient
at room temperature, let alone at almost liquid helium temperature.

You do not write after how many geometry iterations you do not see
significant chage of the pressure, but from your NVP procedure, I assume
you check after a few hundred iterations. For a system with that many
atoms, that is not a lot.

As a general rule of thumb, conjugate gradient relaxation takes about as
many geometry steps as you have degrees of freedom in you system. Of course
this depends on the system and your convergence criteria to some degree,
still I would expect needing anywhere between 1000 and 5000 steps for a
system like yours, for geometry relaxation at constant volume. Even small
differences in the equilibrium bond lengths between whatever force field
you use and the DFTB parameters you use can easily explain your pressure of
10^9 Pa. During the constant volume part of the CG relaxation, drastic
change of the pressure is very unlikely to occur since you use a quite
realistic model from your MM calculations.

You would have to perform an optimization of the lattice vectors to arrive
at a low pressure (Keyword LatticeOpt in the DFTB+ manual). However, the
lattice optimization is performed in its own loop outside the atomic
configuration. That means, that for lattice optimization, geometry
optimizations at constant volume are performed for different volumes and
cell shapes. It can take anywhere between 5 and 20 lattice steps, possibly
even more. It is very difficult to predict if you will need many geometry
steps after a change in the lattice vectors or not. This depends a lot on
the symmetry and rigidity of your system.

Unfortunately, there is not really any way around this procedure, if you
want to have your model at a pressure close to 1 atm. However, it is
probably possible to reduce the number of necessary geometry and lattice
iterations, even if you chose not to optimize the DFTB cell volume:

You could take a much smaller model of maybe 150 atoms or thereabouts,
perform your LAMMPS procedure, then do a full geometry and lattice
optimization in DFTB. From that you can obtain the equilibrium densities of
your model for both force-field and DFTB. If you then rescale the big model
to obtain the same volume ratio between LAMMPS output und DFTB+ input as
for the small model, that will likely save you a lot of geometry iterations.

Hope this helps,

Jan

Jan M. Knaup                      | Fon +49-(0)421-218-62351
Dipl. Phys. Dr. rer. nat.         | Fax +49-(0)421-218-62770
Universität Bremen - BCCMS        |
Am Fallturm 1                     | Jan.Knaup at bccms.uni-bremen.de
28359 Bremen                      | JanKnaup at gmail.com
Germany                           | www.bccms.uni-bremen.de


2012/8/16 bikash sankar kanungo <biku.kanungo at gmail.com>

> Hi,
>
> Hi I am trying to relax an epoxy polymer matrix in DFTB . The polymer
> matrix is obtained by cross-linking the epoxy monomers with a curing agent.
> The cross-linking is done using Classical MD tool LAMMPS. Before feeding
> the cross-linked structure(of 1322 atoms) from LAMMPS to DFTB I ensured to
> keep the pressure to be 1 atm by doing NPT runs in LAMMPS. But while doing
> structure relaxation in DFTB using Conjugate Gradient Driver I do not see
> any change in order of magnitude of pressure which remains as high as
> 1.0E+09 Pa. With a hope of reducing the pressure to 1.0E05(1 atm) I even
> tried Verlocity Verlet using Berendsen Thermostat and Barostat with target
> temperature = 10K , target pressure=1.0E+05 Pa , Timestep=0.1fs, Timescale
> = 50fs but the pressure still remains at around 1.0E+09 Pa even after 10fs.
> To obtain SCC convergence I have set Broyden mixing parameter =
> 0.3(default=0.2\0 and Fermi Fillinf temperature=300K(default=0K). Do I need
> to provide more time for convergence for such a large system or are there
> other alternatives to boost the convergence rate?
>
> I would appreciate any help or suggestions regarding the relaxation of
> this seemingly difficult structure.
>
> Thank you,
> Bikash
> --
> BIKASH SANKAR KANUNGO
> Final Year Undergraduate student,
> Mechanical Engineering Department,
> INDIAN INSTITUTE OF TECHNOLOGY
> KHARAGPUR.
> +919749935409
>
>
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>
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