GW

NOMEGA= integer ！ spcifies the number of frequency grid points

NOMEGA= integer ！ spcifies the number of frequency points along real axis

default： NOMEGA= 50  NOMEGA= NOMEGA for GW。

NOMEGA的取值一般为50-100之间，且必须为CPU核心数数整数倍，

同时NBANDS也必须为CPU核心数整数倍。

如果不是的话，vasp会随机产生波函数，补成整数倍，这些随机产生的波函数会导致结果出现很大误差。

NEDOS  ！number of energy points 可决定介电函数的取值密度

默认NEDOS=2000， 在OUTCAR里介电函数的取值密度是 能量范围除以2000

                 在vasprun.xml里有10倍精细度的介电函数

NBANDSGW=   ！determines how many QP energies are calculated and updated in GW calculations.

This value usually needs to be increased somewhat for partially or fully selfconsistent calculations. Very accurate results are only obtained when NBANDSGW  approaches NBANDS, although this dramatically increases the computational requirements.

BSE

ALGO=BSE

用GW算介电常数，一般是结合BSE一起计算的，算完BSE后在vasprun.xml中有一个精细的介电矩阵。

NBANDSO = 4 ! number of bands for electron-hole treatment (occupied)

NBANDSV = 6 ! number of bands for electron-hole treatment (virtual)

OMEGAMAX=6 ! max frequency

yunhailiseu：

when vasp reports：

————————————————————————————

“BSE diagnonalizing matrix （****）

 BSE calculating oscillator strength”

————————————————————————————

the whole calculation is almost over.

And if no error message is given after that, the job just finished sucessfully .

The BSE  oscillator strength and dielectric tensor is written in vasprun.xml, not OUTCAR。

————————————————————————————

<dielectricfunction>

 <imag>

  <array>

   <dimension dim="1">gridpoints</dimension>

   <field>energy</field>

   <field>xx</field>

   <field>yy</field>

   <field>zz</field>

   <field>xy</field>

   <field>yz</field>

   <field>zx</field>

   <set>

    <r>     0.0000     0.2951     0.2951     0.0068     0.0000     0.0000     0.0000 </r>

    <r>     0.0120     0.2952     0.2952     0.0068     0.0000     0.0000     0.0000 </r>

    <r>     0.0239     0.2952     0.2952     0.0068     0.0000     0.0000     0.0000 </r>

    <r>     0.0359     0.2953     0.2953     0.0068     0.0000     0.0000     0.0000 </r>

————————————————————————————    

Then in the following BSE calculation the excitation energies can be determined ( in vasprun.xml just before the dielectric tensor, together with transition matrix elements ).

If one exciton has a corresponding excitation energy lower than Eg, it is a bound exciton（束缚激子）.

And if the excitation energy is larger than Eg, it is a resonant exciton（共振激子）.

The type of exciton may also be determined from absorption spectrum.

Nano Lett. 2010, 10, 426-431, Nano Lett. 2007, 7, 3112-3115 and PRB 83, 085405 (2011).