Hot answers tagged

33

Below is a summary of what I have found: What is meant by "Exact exchange"$\,$? The question mentions the following (in order of how they were presented in the question): Exact exchange-correlation potential (used in this 2019 paper in Nature Communications). Exact Hartree-Fock exchange energy functional (only 4 results, 3 from the same authors). Exact ...


16

There is linear scaling DFT, where you use density matrix as your object throughout SCF iterations instead of the wavefunctions. The linear scaling comes from the sparsity assumption/truncation on the density matrix. Usually, the wavefunctions are dense vectors, and any eigensolver results in cubic scaling with respect to the number of electrons/bases. ...


13

A few materials/simulation boxes I've had some proper trouble with: HSE06 + noncollinear magnetism + antiferromagnetism, Vasp noncollinear: This was a strongly antiferro material (4 Fe atoms, in an up-down-up-down configuration). HSE06 is apparently difficult to converge anyway. Noncollinear magnetism/antiferromagnetism apparently creates problems for any ...


12

The clearest example in my mind is if you want to understand the orbital-based contributions to some phenomena (e.g. bonding, a reaction energy), particularly if the periodic material being modeled is more like a molecular solid where the chemical picture of orbitals is more intuitive than bands. There are several schemes out there that try to go from PAW to ...


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 ...


10

This will be a long answer, so I will divide it in parts. Woods paper A significant limitation of the Woods et al paper is that it excludes atomic-basis set calculations where convergence acceleration is much more powerful than in plane wave codes. Namely, the update schemes discussed in the article talk about just the input and output densities, whereas ...


10

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 ...


9

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 ...


8

The GPAW code can do GW/BSE and it uses purely PAW potentials. So it's absolutely possible to do GW/BSE with PAW sets. The involvement for the implementation, however, is another topic. From Kevin J. M.'s answer, the primary block is fairly clear: PAWs are quite involved to implement. Doing all of the atom-centered corrections and then getting them onto your ...


8

Whenever you see EXX, a warning flag should go up inside your head. It is never clear whether one means just using Hartree-Fock exchange in a DFT calculation, implying a generalized Kohn-Sham scheme with a fully non-local potential, or if one means an optimized effective potential approach in which the exact Hartree-Fock exchange energy is inverted to find a ...


8

In orbital-free DFT, the key quantity is the particle density (not the density matrix, the actual density). The particle density is a scalar 3D field; when we increase the particle number in our simulation, the values of the particle density change, subject to the constraint that for $N$ particles in a volume $V$ of space, $$ \iiint_V \rho({\bf r})d^3{\bf r} ...


7

For linear scaling both Hamiltonian matrix ($H$) and density matrix ($D$) need to be sparse. If either $D$ or $H$ are dense, then the resulting scaling is quadratic $O(N^2)$. When both $H$ and $D$ are dense, then the scaling is cubic $O(N^3)$. As mentioned above the underlying computational cost is determined by matrix-matrix multiplication. For example, to ...


6

I hope it's okay to "self-answer". So I also posted this question in the GPAW mailing list and one of the users in the mailing list who go by the handle mazay0 offered a different means of installing gpaw as follows: !apt install python3-mpi4py cython3 libxc-dev gpaw-data !pip -q install gpaw pymatgen==2019.12.22 This one worked. Although, I still ...


6

It is well-established that it can be quite difficult to converge the SCF for certain (hybrid) meta-GGA functionals compared to their (hybrid) GGA counterparts. This is especially true for plane-wave periodic DFT and is most often the case with many of the popular Minnesota functionals. In large part because of this reason, a revised version of M06-L ...


5

The question is related essentially to solve the Kohn-Sham equation with an atomic-like basis set. In fact, different implementations of DFT are distinguished mainly by their basis set and how they orthogonalize themselves to the core levels. In particular, the choice of basis set forms the core of any electronic structure method. Dedepending on the choice ...


5

Here's some calculations I had some problems with: LaFeO$_3$ on LaAlO$_3$ with an adsorbed O atom. $\sqrt{2}\times\sqrt{2}$ perovskite cell in the x-y direction. 5 layers of LaAlO$_3$ (alternating LaO - AlO$_2$ - LaO etc.) with 3 layers of LaFeO$_3$ on top. The bottom 3 layers of LaAlO$_3$ are fixed. Anti-ferromagnetic order on the Fe atoms. Around 450 ...


5

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 ...


3

This isn't a perfect comparison of what you asked, but I think it can help. I'll refer to PAW datasets as "pseudopotentials" here since that is how they are used in practice. According to this relatively recent paper, when compared to all-electron calculations, the Standard Solid State Pseudopotential library (which has many PSLibrary ...


2

You can get the induced field components from an induced field object. If you call your induced field object ind, then you can get the Fourier transform of the field components by accessing ind.Fef_wvg. Adding the initial kick strength to this and taking the magnitude you can get the total electric field. Comparing the total electric field magnitude to the ...


2

PAW is proposed to deal with the interaction between electrons and ions. However, PBE or GGA (PBE just one kind of GGA) is just the exchange-correlation functional. You can think PAW and PBE are related to the two different terms in the Kohn-Sham equation. So your question is totally meaningless because you are comparing two completely different concepts.


Only top voted, non community-wiki answers of a minimum length are eligible