Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: First-principles studies of the Optical anisotropy of R3 space group
  chalcogenides crystal AX2MQ6   In order to explain the reason that all the sythesised \ce{AX2MQ6}
chalcogenides compounds are phase-matching while isomorphic chalcogenides
compounds \ce{AX4M5Q12} are not phase-matched materials. The linear optical
property birefringence and non-linear optical property of $R3$ space group
crystals \ce{AX2MQ6} have been calculated to compare with \ce{AX4M5Q12}. Their
Electron Localization Function(ELF) was calculated to show that the acentricity
of the bonds on $sp^3$ hybridization \ce{Q^2-} ions is the main origin of
optical anisotropy. To quantificat the acentricity, a geometric parameter
dihedral angle between tetrahedral undersides and xy-planes was defined. We
find theoretically the birefringence depend on the defined geometric parameter
and ion radius. This relation between birefringence and structure resonalbly
explain the abnormal difference of birefringence of isostructural \ce{AX2MQ6}
and \ce{AX4M5Q12} and useful for exploring new phase-matching IR-NLO crystals.

13
The categories of this paper are: quant-ph
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
28
The categories of this paper are: alg-geom
22
The categories of this paper are: chao-dyn
27
The categories of this paper are: comp-gas
20
The categories of this paper are: acc-phys
12
The categories of this paper are: nucl-th
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
2
The categories of this paper are: math
19
The categories of this paper are: eess
17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg