Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ab initio lattice dynamical studies of silicon clathrate frameworks and
  their negative thermal expansion   The thermal and lattice dynamical properties of seven silicon clathrate
framework structures are investigated with ab initio density functional methods
(frameworks I, II, IV, V, VII, VIII, and H). The negative thermal expansion
(NTE) phenomenon is investigated by means of quasiharmonic approximation and
applying it to equal time displacement correlation functions. The thermal
properties of the studied clathrate frameworks, excluding the VII framework,
resemble those of the crystalline silicon diamond structure. The clathrate
framework VII was found to have anomalous NTE temperature range up to 300 K and
it is suitable for further studies of the mechanisms of NTE. Investigation of
the displacement correlation functions revealed that in NTE, the volume
derivatives of the mean square displacements and mean square relative
displacements of atoms behave similarly to the vibrational entropy volume
derivatives and consequently to the coefficients of thermal expansion as a
function of temperature. All studied clathrate frameworks, excluding the VII
framework, possess a phonon band gap or even two in the case of the framework
V.

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The categories of this paper are: plasm-ph
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The categories of this paper are: funct-an
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The categories of this paper are: acc-phys
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The categories of this paper are: dg-ga
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The categories of this paper are: atom-ph
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The categories of this paper are: hep-th
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The categories of this paper are: nlin
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The categories of this paper are: q-alg
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The categories of this paper are: patt-sol
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The categories of this paper are: eess
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The categories of this paper are: ao-sci
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The categories of this paper are: solv-int
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The categories of this paper are: q-bio
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The categories of this paper are: q-fin
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The categories of this paper are: cmp-lg
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The categories of this paper are: adap-org