# Can excitons at different K points be computed?

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

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

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

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"
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>
``````