在回答上面问题的之前，我绝对有必要了解一下CASTEP计算光学性质的主要原理，CASTEP计算的光学性质主要电子能带结构中最基本的跃迁方式，其他的考虑不多，如声子（晶格振动吸收），激子，自由电子气光学响应等，在CASTEP里面也有这个说明了，比如：
Limitations of the methodLocal field effectsThe level of approximation used here does not take any local field effects into account. These arise from the fact that the electric field experienced at a particular site in the system is screened by the polarizability of the system itself. So, the local field is different from the applied external field (that is, the photon electric field). This can have a significant effect on the spectra calculated (see the example of bulk silicon calculation below) but it is prohibitively expensive to calculate for general systems at present.
Quasiparticles and the DFT bandgapIn order to calculate any spectral properties it is necessary to identify the Kohn-Sham eigenvalues with quasiparticle energies. Although there is no formal connection between the two, the similarities between the Schrödinger-like equation for the quasiparticles and the Kohn-Sham equations allow the two to be identified. For semiconductors, it has been shown computationally (by comparing GW and DFT band structures) that most of the difference between Kohn-Sham eigenvalues and the true excitation energies can be accounted for by a rigid shift of the conduction band upward with respect to the valence band (
Godby et al., 1992). This is attributed to a discontinuity in the exchange-correlation potential as the system goes from (N)-electrons to (N+1)-electrons during the excitation process. There can, in some systems, be considerable dispersion of this shift across the Brillouin zone, and the scissor operator used here will be insufficient.
Excitonic effectsIn connection with the absence of local field effects, excitonic effects are not treated in the present formalism. This will be of particular importance for ionic crystals (for example NaCl) where such effects are well known.
Other limitations
The nonlocal nature of the GGA exchange-correlation functionals is not taken into account when evaluating the matrix elements but it is expected that this will have a small effect on the calculated spectra.
Phonons and their optical effects have been neglected.
Finally, there is an intrinsic error in the matrix elements for optical transition due to the fact that pseudowavefunctions have been used (that is they deviate from the true wavefunctions in the core region). However, the selection rules will not be changed when going from pseudo- to all-electron wavefunctions
比如第一条所说的局域场效应，我们在计算光学跃迁的时候，外界跃迁激发电场在材料内部认为是没有衰减的，实际上由于内场的作用，一部分电场会被Screen了，但我们没有考虑。其次提到了，DFT计算的单粒子激发谱方面存在的低估问题，这个可以通过一个对能带的刚性平移实现，也就是常说的剪刀工具。上面也提到了赝势，Exc等效应对光学性质的影响。
中所周知的是Optical Properties （OP）计算主要是从复合介电方程开始的， 介电方程中虚部表示了和能带之间跃迁有关的信息，峰值可能和The First Brillouin Zone的Van－HOff singularity 有关系，现在DFT计算结构比较复杂，要解析这些关系，即具体的解析出BZ结构中不同k点附近的Van－Hoff奇点是很困难的，不过在20世界50年代以后的很多文献对一些简单的半导体结构做了计算，如As，Si，AsP等，这些物质晶体结构比较简单，因此可以比较详细的了解Van－Hoff奇点到底在BZ区那个位置。
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一志愿厦门大学化学工程（专硕）-数二英二406分-求调剂
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