[DFTB-Plus-User] Slow Convergence problem

bikash sankar kanungo biku.kanungo at gmail.com
Thu Aug 16 18:25:37 CEST 2012 

Dear Jan,

Thanks for such a descriptive and insightful reply. I kept the timestep
=0.1 fs as the temperature of the system showed a sharp rise for 1fs
ultimately printing Nan. I made some trials and found 0.25fs to be the the
maximum timestep that worked for me.


For my NPT(Berendsen thermostat and barostat) run in DFTB I did not notice
any pressure change after 100 geometry steps( = 10fs for a timestep of
0.1fs). But since I have set Timescale=50fs so after 10fs I expected some
change in pressure. May be I am being too anxious about my outputs.

I am doing a LatticeOpt simultaneously , but strangely I never got beyond
Lattice Step = 0 for any of the 100 Geometry Steps I have reached so far.
Is that an expected behavior ?

For the time being I will let them go upto 1000-2000 steps which may take a
week and see if I get any success else I will have to for a smaller system
and work out the re-scaling approach you suggested.

Thank you,
Bikash

On Thu, Aug 16, 2012 at 8:51 PM, Jan M. Knaup <Jan.Knaup at bccms.uni-bremen.de
> wrote:

> 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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-- 
BIKASH SANKAR KANUNGO
Final Year Undergraduate student,
Mechanical Engineering Department,
INDIAN INSTITUTE OF TECHNOLOGY
KHARAGPUR.
+919749935409
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