[DFTB-Plus-User] Reliability of prediction on conduction band of silicon nano crystals

ZHAOHUI HUANG zuh101 at psu.edu
Tue Feb 21 19:10:45 CET 2017


If somebody wants to use, I can give him a copy, but he needs to prepare for TB parameters first.  My Si-Si parameters are taken from a PRB paper in 2007(?), where they did not include non-orthogonal effect so here I simply solve the eigen-problem AX=aX rather than AX=aMX. It is easy to include such correction by add a few statements in reverse communication interface, of ARPACK, if necessary.

Can I continue to ask a question, when handling higher conduction bands, what is wrong with sp3d5s* basis? wrong eigenvalue? wrong velocity or wrong effective mass? no hope to employe TB method to predict optical properties? thanks.

----- Original Message -----
From: "Ben Hourahine" <benjamin.hourahine at strath.ac.uk>
To: dftb-plus-user at mailman.zfn.uni-bremen.de
Sent: Tuesday, February 21, 2017 12:40:22 PM
Subject: Re: [DFTB-Plus-User] Reliability of prediction on conduction band of silicon nano crystals

Hello ZhaoHui Huang,

Your TB code sounds very interesting, do you have plans to release it in
some form?

For quantum mechanical models, the total electronic energy (and as a
result the forces) requires functions of all the valence states, not
just those near to the band-edges. For DFTB there is also the
complication that this is a non-orthogonal basis, so requires
generalised eigenstates (i.e., orthogonal under the action of the
overlap matrix).

For empirical tight binding, what can be done is to relax geometries
with classical inter-atomic potentials, and then to solve for the band
edge states of the resulting structure (I believe this approach has been
previously used for quantum dots). In this case something like
bond-order potentials might potentially be a good choice to calculate
the forces. These are derived from tight-binding hopping elements so it
might be possible to make these more consistent with the electronic
structure hamiltonian.

Regarding the conduction bands, sp3d5s* should be able to reproduce the
lower conduction states well, depending on the parametrization. These
bands dominate the response properties, but as shown with the time
dependent DFTB energy and oscillator window methods, higher energy
states can also substantially contribute to response behaviour
(depending on the system).

Regards

Ben

On 21/02/17 14:33, ZHAOHUI HUANG wrote:
> Hello,
>
>     Sorry to bother you if not interested.
>
>     Last fall semester, I sent a message to DFTB+ list and ask if it is possible to solve large size TB Hamiltonian. the feedback said the largest possible size of H is roughly 60,000 for LAPACK routine. Since my silicon mesoscale crystal can easily run up to a few hundreds of thousand of atoms, like 100,000 silicon atoms, I cannot use DFTB+, so I write a MPI TB code with sp3d5s* basis to calculate, based on implicitly restarting arnoldi method (IRAM) as well as intel distributed LU factorization for sparse matrix. I aim at only achieving bands near band gap. Now it works. Here I want to ask DFTB+ community again,
>
>    What is the algorithm to relax the structure? see if I can embed my code into DFTB source, if DFTB relaxation is based on Lanczos method or its derivatives or whatever based on invariant space approach.
>
>    For the above basis, how reliable are prediction of conduction bands, except the lowest one? If I use TB conduction bands, a few thousands, is it possible to calculation response function for my mesoscale crystal? like epsilon, based on TB conduction bands. comments are welcome! thanks a lot 
>
>
> ZhaoHui Huang,
>
>
>
>  
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-- 
      Dr. B. Hourahine, SUPA, Department of Physics,
    University of Strathclyde, John Anderson Building,
            107 Rottenrow, Glasgow G4 0NG, UK.
    +44 141 548 2325, benjamin.hourahine at strath.ac.uk

2013/4 THE Awards Entrepreneurial University of the Year
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