# Tag Info

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Exhibit 1: Ground state hyperfine splitting of the H atom: 1420405751767(1) mHz (present most accurate experiment) 142045199 mHz (present most accurate theory) The error in the theory is due to the difficulty in treating the nuclear structure (2 up quarks + 1 down). Exhibit 2: Ground state hyperfine splitting of the muonium atom: 4463302780(050) Hz (...

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Here is a paper reviewing charge transport in molecular junctions: J. Chem. Phys. 148, 030901 (2018), http://dx.doi.org/10.1063/1.5003306 A typical way to treat such a problem is to couple the Non-equilibrium Green's function (NEGF) formalism with density-functional-theory (DFT), short NEGF-DFT. You need to solve $$(E-H-\Sigma^{R,B}(E))\cdot G^R(E) = I,$$ ...

17

What you are referring to is called band folding. Remember we are plotting the band structure in the reciprocal space. As the size of the cell in real space increases (eg: when you make a super cell), the first Brillouin zone in reciprocal space shrinks and more lines populate the band structure resulting from folding back of lines at the ...

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Unlike non-periodical DFT codes, Quantum ESPRESSO uses planewave basis sets and pseudopotentials. There are many ways of writing your input file, and it all depends on what information you have or what you want to learn. Here is the skeleton of your input file: =============================================================================== &CONTROL ...

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The main positives: you can do all-electron calculations you don't need to set up pseudopotentials / PAWs you can study core properties you can use hybrid functionals cheaper / run post-HF calculations The negatives: basis set is geometry dependent, so you get superposition error it's harder to get results close to the complete basis set limit Either ...

13

To respond directly regarding the Materials Project data, I'm a staff member there so maybe I can shed some light. Materials Project computed data is currently generated using a technique known as Density Functional Theory (DFT) with the PBE exchange-correlation functional. This results in some well-understood, systematic differences from experiment. ...

12

I can't say much about the popularity of GPUs in practical calculations. From a development point of view the speedups that can be expected from GPUs in plane-wave DFT are only moderate, probably around 2 to 3, maybe 7 if you are optimistic. See for example the paper describing the VASP implementation [1] or this stackoverflow question illustrating ...

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Pure plane-wave basis sets have the following advantages when used in periodic DFT (or HF) simulations: Orthogonal Computationally simple (operators with derivatives are particularly straightforward) Low-scaling methods allow easy transformations between real- and reciprocal-space Basis set size does not scale with electron count Independent of atomic ...

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You can start with following this Quantum Espresso Tutorial. The tutorial includes Hands On: scf, Energy bands, eqn of state, Geometry, cell optimization, Magnetism, DFT + U, CPMD, TDDFT, GW, Phonos, Berry Phases, IR and Raman You can also follow Open online course on DFT, with simultaneous hands-on training on Quantum ESPRESSO. Available at any time for ...

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One important property of atom-centered basis sets is that electrons can only be localized on atoms. This is a problematic property when modeling solid systems with defects. For instance, at a color center, an electron is localized at a vacancy site. How can you model this with atom-centered basis sets? You have place a ghost atom at the vacancy site, ...

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Regarding input file format, check this website: https://www.materialscloud.org/work/tools/qeinputgenerator Format from this website will provide you a starting point. But please remind that the result from this website IS NOT the ultimate solution and please do NOT regard the result from here as the "OK-button" solution. It seems that the example in your ...

10

I will give a quick answer from my experience, but typically you want to use the computed lattice parameter. The reason for this is that otherwise you induce strain in your computed cell, but in general this is a red flag anyways. You really want your computed lattice parameter to match if possible which might involve using a different functional or even ...

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While I can't comment on concrete speed-ups as I'm not very familiar with GPU programming itself, I would like to point out that most of the computing time is spent in FFT and GEMM (matrix matrix multipicaltions) calls. A friend of mine tested this for the GPAW code where those two thing accounted for > 70 % of the CPU time. I imagine it is similar for other ...

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The workflow transferring from VASP to CASTEP is quite similar. Within VASP, there are four main files that you need for a calculation: POSCAR – gives the positions of the atoms within the unit cell and its lattice vectors. KPOINTS – sets the k-point mesh to be used. POTCAR – determines the type of pseudopotential to be used. INCAR – controls most of the ...

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It is possible to approximate the cell parameters by determining the convex hull of the molecule and then computing the smallest parallelepiped that encloses it. This will automatically give the dimensions together with the vertices of the parallelepiped from which the angles can be computed. This problem was first investigated by Vivien and Wicker (2002)1, ...

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I experienced[1] about 5-10 times speedup with GPU accelerated Quantum Espresso on Tesla V100 32 Gb compared with Intel Core i7 9700K processor with 8 cores and 32 Gb RAM. The above mentioned system's volume was about 125 cubic Angstroms, it had about 19 atoms, 5 k-points, ecutwfc = 80 Ry, ecutrho = 320 Ry. 24 SCF iterations took 54 seconds with GPU, and ...

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As the answer above mentioned, the biggest difference is perhaps CASTEP takes two input files with suffix "cell" and "param", other things are very similar. After all, both programs do plane-wave pseudopotential calculations. Below are a few other subtle differences (or tips?) - I am a CASTEP user recently started using VASP! In CASTEP ...

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Here's another approach for strongly correlated systems: Al-Hassanieh et al. Phys. Rev. B 73 195304 2006. In this case, they use time-dependent DMRG. They treat the wire as a chain of sites occupied by spinless Fermions and the junction as an impurity in the wire. They can't observe a steady state current because their system has hard boundaries (the wires ...

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I have to disagree with the answer of @Tristan. The x-ray diffraction, both powder and single crystal, has enough precision and quality (equipment and software) to give very good and real experimental values. It is well known that very few DFT approximations (methods, functional, pseudopotential, basis set, etc.) give good lattice values. And when we said ...

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There some very important points to address here. In a periodical structure, the atom coordinates and the cell parameters are not randomly selected. The crystal structure (atom positions, cell parameters, spacial group) are determined by X ray/neutron crystallographic using single crystal or powders. The atoms are placed in the so called Wyckoff positions. ...

7

You have two different question in the same post. how can I simulate the powder XRD spectra? To just simulate the XRD spectra you can use this answer https://mattermodeling.stackexchange.com/a/146/24. can I use the powder XRD to further refine the lattice constants of the structure? The short answer: yes. The long answer: For the fitting process, or ...

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I recently installed VASP GPU version provided by NVIDIA (here's the installation tutorial) on my machine that has an RTX 2080 Ti GPU. GPU version was announced for vasp-5.4.1 and works fairly well for vasp-5.4.4 too. However, I have only managed to observe around 1.5x - 2x speedup when running VASP on the GPU compared to my Intel(R) Core(TM) i7-8700 CPU ...

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