Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Structure and vibrational properties of methane up to 71 GPa   Single-crystal synchrotron X-ray diffraction, Raman spectroscopy, and first
principles calculations have been used to identify the structure of the
high-pressure (HP) phase of molecular methane above 20 GPa up to 71 GPa. The
structure of HP phase is trigonal R3, which can be represented as a distortion
of the cubic phase B, previously documented at 7-15 GPa and confirmed here. The
positions of hydrogen atoms in HP phase have been 19 obtained from first
principles calculations. The molecules occupy four different crystallographic
sites in phases B and eleven sites in the HP phase, which result in splitting
of molecular stretching modes detected in Raman spectroscopy and assigned here
based on 22 a good agreement with the Raman spectra calculated from the first
principles.

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The categories of this paper are: atom-ph
13
The categories of this paper are: quant-ph
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The categories of this paper are: ao-sci
22
The categories of this paper are: chao-dyn
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The categories of this paper are: q-fin
14
The categories of this paper are: math-ph
12
The categories of this paper are: nucl-th
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The categories of this paper are: plasm-ph
19
The categories of this paper are: eess
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The categories of this paper are: supr-con
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The categories of this paper are: stat
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The categories of this paper are: funct-an
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The categories of this paper are: nlin
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The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
7
The categories of this paper are: hep-th