Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: New stable crystal structures of C and GeC2 predicted from
  first-principles calculations   Two novel three-dimensional (3D) crystal structures of carbon (C) and
germanium carbide (GeC2) were predicted using first-principles
density-functional theory (DFT) calculations. These newly discovered 3D carbon
allotrope and GeC2 are in the space group of Fmmm (space group number 69).
Their crystal structures have unique tetragonal/hexagonal rings formed by
either C or Ge/C atoms. Both structures were proven to be thermodynamically
stable through the phonon spectrum calculations. The C allotrope is a
semiconductor with a wide band gap of 3.65 eV predicted by the hybrid density
functional HSE06 method, while GeC2 is metallic. The new C allotrope has a low
mass density of 2.84 g/cm3. More importantly, it is proven to be energetically
stable with cohesive energy less than -7.5 eV/atom which is lower than many
other carbon allotropes implying the possibility to be fabricated in lab. Such
a carbon crystal structure with a low mass density and wide band gap once
synthesized would have wide applications in gas adsorption sensors and
photo-electronic devices.

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The categories of this paper are: q-alg
20
The categories of this paper are: acc-phys
17
The categories of this paper are: q-fin
36
The categories of this paper are: cmp-lg
1
The categories of this paper are: cs
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
31
The categories of this paper are: ao-sci
0
The categories of this paper are: hep-ph
21
The categories of this paper are: adap-org
14
The categories of this paper are: math-ph
32
The categories of this paper are: atom-ph
35
The categories of this paper are: mtrl-th
26
The categories of this paper are: bayes-an
9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph