30

In an era of ab initio methods and many-body methods like the $GW$, there is not too much room for methods like the extended Hückel model to be the main method in any particular field of materials modeling. However, the method is still very much appreciated by the solid-state community particularly for it's accessibility. It is especially popular with those ...


18

One area where the extended-Huckel method continues to see use is to form the initial guess for an SCF calculation or even just a more accurate semi-empirical method. While most electronic structures packages use the Superposition of Atomic Densities (SAD) guess as the default, the option is available in almost all of them and Psi4 uses it as the default for ...


16

After a little research I found a great article [1], which provides a good overview to what I asked above in Figure 2. Summarising in a table: | Method | effort | reliability | system size | +------------------------+--------+--------------+-------------+ | Interatomic potentials | high | high* / low* | 10^8 | | Linear-scaling DFT ...


14

I know of at least one place, where it is relatively common to use the extended Hückel theory in practise: in generating initial guess orbitals for further electronic structure calculations. The most popular example I can think of is Turbomole, see its manual (pdf, chapter 4.3, p. 75). They claim that the starting vectors are better than a core Hamiltonian ...


8

DFTB+ One option is DFTB+. It is free, open source, has been around for more than a couple years now, and has a fairly big community. You are also very lucky, since your question is about the band structure of TiO$_2$ and the sample input that DFTB+ provides for band structure calculations is for... TiO$_2$ :) The following might also be useful: DFTB+ ...


8

In the Huckel method, you are just generating a very simplified version of the molecular Hamiltonian and determining it's eigenvalues. The molecular Hamiltonian will always be a Hermitian matrix (for the Huckel method, we can be more specific and say it's a real symmetric matrix by construction). Hermitian matrices are guaranteed to have all real eigenvalues ...


8

I am sure there are A LOT of authors publishing papers to answer this very question. Mainly because the theories employed have face a paradigm since "one-size-doesn't-fit-all". Here are the variables that affect this use of a certain method: (i) Molecular models or periodic solids (ii) Chemical Accuracy (energies with accuracy of < 1kcal/mol) - e.g. ...


7

ASE has a FixSymmetry constraint that preserves spacegroup symmetries. It works with a variety of structure optimization algorithms. You could use LAMMPS as the engine or one of the many other calculators, including some DFT options.


5

Based on your input file, I tried to run the calculation in my machine. As you didn't specified how did you obtain the original structure (assembled by hand or downloaded from structure database), I first generated a SDF file of your structure with Avogadro, and did a conformer search with obconformer from Open Babel, just to be sure to pick a initial ...


5

If you look at your last five iterations, you will see that the density change (which is what matters most for SCF convergence) is getting smaller in each iteration, meaning that the calculation is still converging, and will converge if you give it more iterations: Iteration SCF Energy Energy change Density Change 26 -2246.3335762326 0.0000000384 0....


5

GULP GULP (General Utility Lattice Program) uses specific forcefields for different type of systems and it is designed to work with periodic conditions. Also, it has defined several type of potential models for two-body (Buckingham, Lennard-Jones, Morse, etc.), three-body (Three-body harmonic, Axilrod-Teller, Stillinger-Weber, etc.), four-body, six-body and ...


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