Can excitons at different K points be computed?

exciting
#1

Hi Exciting users
I have followed the tutorial, but I don’t know if the Exciting code can calculate the excitons at different K points (Such as the hole locates at the M point, and the electron locates at the Gamma point). Can q-dependent BSE cacluate this property?

Best regards

#2

Hi ttbook,

The exciting code is able to do q-dependent BSE but it is not possible to locate the electron and hole at a specific point in the reciprocal space directly.

Though you could try the following workaround:

The element qpointset can be used for q-dependent BSE. Here you specify all q-points to calculate. Technically, it displaces the occupied states about +q/2 and the unoccupied states about -q/2.
The attribute vkloff sets up an offset to the k-points.
Thus the first k-point for the occupied states is vkloff / N_k + q/2 and for the unoccupied states
vkloff / N_k - q/2.

If you want to calculate excitons where the hole locates at the M point and the electron at the Gamma point you could do the following:

  1. Set the Number of k-points (ngridk and ngridq in the xs element) to 1 1 1

  2. Set a qpoint element in the qpointset element to q = Gamma - M

  3. Set vkloff to ** - (Gamma + M) / 2**

The occupied stats (where the hole sits) are now calculated at the k-point
vkloff / N_k + q/2 = -(Gamma + M) / 2 + (Gamma - M) / 2 = M*,

And the unoccupied states (where the electron sits) are calculated at the k-point
vkloff / N_k - q/2 = -(Gamma + M) / 2 - (Gamma - M) / 2 = -Gamma = Gamma*.

Is this what you need? Also, I’ve never tried this, so I can not guarantee good results!!!

Cheers

#3

Hi maurerben, thanks for your reply, I will try to calculate it first.

#4

Dear maurerben,
I did the tests according to the methods you suggested. When set the Number of k-points (ngridk and ngridq in the xs element) to 1 1 1, no documents related to EPSILON and LOSS were produced. The EXCCOEFF_BSE-singlet-TDA-BAR_SCR-full_QMT001.OUT also was not produced. The input.xml has been pasted.

Best regards

<input>
   <title>Graphene</title>
     <structure speciespath="/public1/home/sch3140/soft/exciting/species">
       <crystal scale="4.65">
         <basevect> 1.0 0.0000000000 0.0000 </basevect>
         <basevect> -0.5 0.8660254040 0.0000 </basevect>
         <basevect> 0.0 0.0000000000 10.077 </basevect>
       </crystal>

       <species speciesfile="B.xml" rmt="1.2805">
         <atom coord="0.3333333333333357  0.6666666666666643  0.50"/>
       </species>
       <species speciesfile="N.xml" rmt="1.3296">
         <atom coord="0.6666666666666643  0.3333333333333357  0.50"/>
       </species>
      </structure>

      <groundstate
        do="fromscratch"
        xctype="GGA_PBE_SOL"
        ngridk="8 8 1"
        rgkmax="4"
        swidth="0.01"
        nosym='true'>
      </groundstate>

      <properties>
        <bandstructure>
          <plot1d>
            <path steps="100">
              <point coord="0.0 0.0 0.0" label="GAMMA"/>
              <point coord="0.5 0.0 0.0" label="M" />
              <point coord="-0.33333333 -0.33333333 0.0" label="K" />
              <point coord="0.0 0.0 0.0" label="GAMMA"/>
            </path>
          </plot1d>
        </bandstructure>
      </properties>

     <xs
        xstype="BSE"
        ngridk="1 1 1"
        vkloff="0.097 0.273 0.03"
        ngridq="1 1 1"
        nempty="30"
        gqmax="3.0"
        broad="0.007"
        scissor="0.0001"
        tappinfo="true"
        tevout="true">

      <energywindow
        intv="0.0 1.0"
        points="1200"/>

      <screening
        screentype="full"
        nempty="100"/>

      <BSE
          bsetype="singlet"
          nstlbse="1 4 1 4"/>
      <qpointset>
          <qpoint> 0.0 0.0 0.0</qpoint>
      </qpointset>

      <storeexcitons
         selectenergy="false"
         MinNumberExcitons="1"
         MaxNumberExcitons="20"
         MinEnergyExcitons="0"
         MaxEnergyExcitons="13.8"/>

   </xs>

 </input>