Register to reply

Quastion about K-points and energy cutoff

by omaralrawi
Tags: cutoff, energy, kpoints, quastion
Share this thread:
omaralrawi
#1
May2-14, 03:08 PM
P: 1
Dear All,

I have three questions about the relation between the K-points and energy cutoff. The first one is : how many k points should I use in my theoretical calculation to get a good result in lattice constant and the converge of Emin especially for using DFT? The second one is : What is the relation in K-point and energy cutoff and is there any equation that can I use to describe this relation? How can I calculate this relation for graphene?

I have used in my calculation GaAs to gain an accurate result.

Regards.
Phys.Org News Partner Physics news on Phys.org
Working group explores the 'frustration' of spin glasses
New analysis of oxide glass structures could guide the forecasting of melt formation in planetary interiors
Scientists characterize carbon for batteries
bsmile
#2
May6-14, 01:02 AM
P: 27
Which software are you using? Seems like planewave based DFT calculation, I guess.

(1) if using pseodupotential, then less K points can be used as compared with all-electron calculations. For VASP and insulator calculation, 4*4*4 might be good enough. Of course you can always use more K points to test K point convergence. You can refer to the vasp manual to find more information

(2) Energy cutoff has nothing to do with K point. Energy cutoff is related to so-called G points, plane wave basis, and their relationship is simply E_cutoff=h_bar^2 G_cutoff^2/2m_e.

(3) Energy cutoff is different from the dispersion relation relating K to E. Energy cutoff in a DFT software is more a technical trick than a physically meaningful quantity.
Douasing
#3
May20-14, 03:26 AM
P: 33
Quote Quote by bsmile View Post
Which (1) if using pseodupotential, then less K points can be used as compared with all-electron calculations. For VASP and insulator calculation, 4*4*4 might be good enough.
Here is a question making me feel puzzled.
Actually,I don't know very clear why 4*4*4 might be good enough for insulator,especially for atomic or molecular system,generally, 1*1*1 is enough.
For k points (lowercase,k),it should perform the periodic boundary condition. In my opinion,the number of k points is just equal to N1*N2*N3. Whatever the system is insulator or molecular,they need to be calculated through periodic boundary condition (for molecular,say,supercell).
Because the calculation is always made in a primitive cell (or,supercell),in other words, the number of k points stands for the number of primitive cells. But ,Why should the number of k-points for metals be more than insulator or semicoductor ? And why 1*1*1 for molecular (just one k point)?

bsmile
#4
May20-14, 10:51 AM
P: 27
Quastion about K-points and energy cutoff

The VASP software manual might address your question better. For insulator, there is no band around Fermi surface, thus K space sampling can be sparse.

For atom and molecule calculation, you do only need 1*1*1 with a very big unit cell, which means two nearby atoms/molecules are separated faraway such that their interaction is tiny. Why 1*1*1 is enough is because the K dependence of density/wavefunction/potential has very weak dependence on K (imagine the dispersion for a single atom in K space, which is basically flat).

I think lowercase k in your language is similar to G in my language (VASP). We might use the same uppercase K to denote crystal momentum within the 1st Broullin zone.


Register to reply

Related Discussions
Quastion about abelian group Calculus & Beyond Homework 7
Mesh cutoff energy Atomic, Solid State, Comp. Physics 1
Planewave cutoff energy convergence! Atomic, Solid State, Comp. Physics 4
Quastion in Biology Biology, Chemistry & Other Homework 2
I have a quastion General Physics 11