29
votes
Accepted
How accurate are the most accurate calculations?
Exhibit 1: Ground state hyperfine splitting of the H atom:
...
- 33.6k
25
votes
Accepted
Why is the band structure of a supercell more dense than for simple cell?
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 ...
- 8,922
18
votes
Accepted
How are charge transfer processes modeled in materials?
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-...
- 1,401
17
votes
Accepted
How to do periodic DFT calculations using Quantum ESPRESSO?
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 ...
- 4,719
17
votes
What are the positives and negatives of periodic DFT codes that don't use plane-wave basis sets?
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 ...
- 18k
16
votes
When do I use the lattice parameters obtained computationally vs experimentally?
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 ...
- 1,501
15
votes
Accepted
What are the positives and negatives of periodic DFT codes that don't use plane-wave basis sets?
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)
...
- 6,647
14
votes
What are the positives and negatives of periodic DFT codes that don't use plane-wave basis sets?
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 ...
- 323
13
votes
How to do periodic DFT calculations using Quantum ESPRESSO?
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, ...
- 8,922
12
votes
How to do periodic DFT calculations using Quantum ESPRESSO?
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 ...
- 843
12
votes
What is the status of graphics processing units in plane-wave DFT?
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 ...
- 2,384
12
votes
Moving from VASP to CASTEP for bulk crystal and surface calculations
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 ...
- 701
11
votes
Accepted
Moving from VASP to CASTEP for bulk crystal and surface calculations
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 ...
- 398
10
votes
When do I use the lattice parameters obtained computationally vs experimentally?
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 ...
- 10.8k
10
votes
What is the status of graphics processing units in plane-wave DFT?
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 ...
- 898
9
votes
What is the status of graphics processing units in plane-wave DFT?
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 ...
- 505
8
votes
How are charge transfer processes modeled in materials?
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 ...
- 6,579
8
votes
Is there any Python library for returning the atoms of a periodic material within a cube?
You can use a python library called ASE (Atomic Simulation Environment) to achieve your requirement.
Here is a sample code:
...
- 2,895
8
votes
Accepted
Pattern X-Ray Diffraction Fitting
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/...
- 22.3k
8
votes
When do I use the lattice parameters obtained computationally vs experimentally?
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 ...
- 22.3k
8
votes
Accepted
Guess cell parameters from atomic coordinates
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 ...
- 1,607
6
votes
Unit Cell Parameters for VASP Calculations
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 ...
- 22.3k
5
votes
Accepted
What is the status of graphics processing units in plane-wave DFT?
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 ...
- 1,962
5
votes
Accepted
Transition dipole matrix element
Expanding my comment to an answer, I believe the "trick" is incorrect.
As far as I'm aware, the relationship between $\boldsymbol{r}$ and $\boldsymbol{\nabla_k}$ is $$\boldsymbol{r}\to-i\exp(...
5
votes
What is the difference between a "periodic image" and a "nearest image" in MD simulation?
You can generate a periodic image n times in any direction. In comparison, the nearest image is the one that is calculated as +x or -x in the x-direction, +y or -y in the y-direction and +z or -z in ...
- 2,772
5
votes
Accepted
Screened Coulomb interaction and its direct Fourier transform
The following is not mathematical rigorous. Most importantly, I want to stress that it is hard to make precise statements without knowing the conventions and definitions of the source (which doesn't ...
- 659
4
votes
What is the difference between a "periodic image" and a "nearest image" in MD simulation?
In the general case, we have lattice vectors ${\bf L}_{i}$, which may not be orthogonal. With periodic boundary condition, a particle at ${\bf r}$ has periodic images at ${\bf r}+n_1{\bf L}_1+n_2{\bf ...
- 18k
4
votes
Accepted
Plane-wave basis expansion for periodic material modelling: cutoff energy inequality and number of atoms in the simulation
You’re quite right: as written, the equation is dimensionally inconsistent. There are actually a bunch of constants that make the units work out, but it is customary in matter modeling to define units ...
- 776
4
votes
Large energy differences across successive geometry optimization steps in SIESTA
In order to calculate the adsorption energies of cetophenone on a nickel surface, you need to do three calculations.
Considering the same cell parameters:
Run a calculation with only the nickel ...
- 22.3k
3
votes
Electric quadrupole operator for periodic systems
To my understanding, the (approximate) substitution to transform the dipole from real to k-space is:[1] $$\mathbf{r}\to i\exp(i\mathbf{k}\cdot \mathbf{r})\nabla_k\exp(-i\mathbf{k}\cdot \mathbf{r})\tag{...
- 15.2k
Only top scored, non community-wiki answers of a minimum length are eligible