Difference between revisions of "Tutorial:Wires and slabs"

From OctopusWiki
Jump to navigation Jump to search
Line 1: Line 1:
 
Under construction
 
Under construction
 
= Ground state calculation =
 
= Ground state calculation =
Hexagonal boron nitride (HBN) is an insulator widely studied which has a similar structure to graphene. Here we will describe how to get the band structure of an HBN monolayer.  
+
Hexagonal boron nitride (h-BN) is an insulator widely studied which has a similar structure to graphene. Here we will describe how to get the band structure of an h-BN monolayer.  
 
A sheet of HBN is periodic in the x-y directions, but not in the z. Thus we will set {{variable|PeriodicDimensions|System}} = 2  
 
A sheet of HBN is periodic in the x-y directions, but not in the z. Thus we will set {{variable|PeriodicDimensions|System}} = 2  
  

Revision as of 10:28, 16 November 2017

Under construction

Ground state calculation

Hexagonal boron nitride (h-BN) is an insulator widely studied which has a similar structure to graphene. Here we will describe how to get the band structure of an h-BN monolayer. A sheet of HBN is periodic in the x-y directions, but not in the z. Thus we will set PeriodicDimensions = 2

here BN lenght= 1.445*angstrom 'L' large enough to describe a monolayer in the vacuum. One should always converge the box length value. Here is the inp file for the GS calculation:


CalculationMode = gs

FromScratch = yes

ExperimentalFeatures = yes

PeriodicDimensions = 2

Spacing = 0.20*angstrom

BoxShape = parallelepiped


BNlength = 1.445*angstrom
a = sqrt(3)*BNlength
L=40

%LatticeParameters
 a | a | L
%


%LatticeVectors
 1    | 0         | 0.
 -1/2 | sqrt(3)/2 | 0.
 0.   | 0.        | 1.
%
 
%ReducedCoordinates
 'B' | 0.0 | 0.0  | 0.00
 'N' | 1/3 | 2/3  | 0.00
% 


PseudopotentialSet=hgh_lda

LCAOStart=lcao_states 


%KPointsGrid
  12   | 12   | 1
%

ExtraStates = 5

UnitsOutput = ev_angstrom


Band Structure

Convergence of the band gap with respect to the spacing for a HBN monolayer.


After this GS calculation we will perform an unocc run. This non-self consistent calculation which needs the density from the previous GS calculation.


CalculationMode = unocc


Mettre warning du log: mode de caclule en BS: pas d reecriture des fonction du GS.


In order to calculate the band structure along a certain path along the BZ, we will use the variable KPointsPath . Instead of using the KPointsGrid block of the GS calculation, we use during this unocc calculation:


%KPointsPath
 12  |  7  | 12                 # Number of k point to sample each path
 0   |  0  | 0                  # Reduced coordinate of the 'Gamma' k point
 1/3 | 1/3 | 0                  # Reduced coordinate of the 'K' k point
 1/2 |  0  | 0                  # Reduced coordinate of the 'M' k point
 0   |  0  | 0                  # Reduced coordinate of the 'Gamma' k point
%


The first row describes how many k points will be used to sample each segment. The next rows are the coordinate of k points from which each segment start and stop. In this particular example, we chose the path: Gamma-K, K-M, M-Gamma using 12-7-12 k points. In Figure 1 is plotted the output band structure where blue lines represent the occupied states and the reds one the unoccupied ones.

One should also make sure that the calculation is converged with respect to the spacing. Figure 2 shows the band gap for several spacing values. We find that a spacing of 0.14 Angstrom is needed in order to converge the band gap up to 0.01 eV.