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Liquids are in thermal equilibrium and have a non-zero structure factor 
= ![[\langle N^2\rangle - \langle N\rangle^2]/\langle N\rangle](//journals.iucr.org/a/issues/2010/01/00/zm5067//teximages/zm5067fi2.gif)
= 
in the long-wavelength limit where 
is the number density, 
is the temperature, 
is the scattering vector and 
is the isothermal compressibility. The first part of this result involving the number 
(or density) fluctuations is a purely geometrical result and does not involve any assumptions about thermal equilibrium or ergodicity, so is obeyed by all materials. From a large computer model of amorphous silicon, local number fluctuations extrapolate to give 
= 0.035 
0.001. The same computation on a large model of vitreous silica using only the silicon atoms and rescaling the distances gives = 
, which suggests that this numerical result is robust and perhaps similar for all amorphous tetrahedral networks. For vitreous silica, it is found that = 
, close to the experimental value of = 
obtained recently by small-angle neutron scattering. Further experimental and modeling studies are needed to determine the relationship between the fictive temperature and structure.
= ![[\langle N^2\rangle - \langle N\rangle^2]/\langle N\rangle](http://journals.iucr.org/a/issues/2010/01/00/zm5067//teximages/zm5067fi2.gif){kind=small}
= 
in the long-wavelength limit where 
is the number density, 
is the temperature, 
is the scattering vector and 
is the isothermal compressibility. The first part of this result involving the number 
(or density) fluctuations is a purely geometrical result and does not involve any assumptions about thermal equilibrium or ergodicity, so is obeyed by all materials. From a large computer model of amorphous silicon, local number fluctuations extrapolate to give 
= 0.035 
0.001. The same computation on a large model of vitreous silica using only the silicon atoms and rescaling the distances gives = 
, which suggests that this numerical result is robust and perhaps similar for all amorphous tetrahedral networks. For vitreous silica, it is found that = 
, close to the experimental value of = 
obtained recently by small-angle neutron scattering. Further experimental and modeling studies are needed to determine the relationship between the fictive temperature and structure.
