Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: First-principles theory of coloration of WO$_3$ upon charge insertion   We report first-principles investigations of the coloration of WO$_3$ upon
charge insertion, using sodium tungsten bronze (Na$_x$WO$_3$) as a model
system. Our results explain well the systematic color change of Na$_x$WO$_3$
from dark blue to violet, red-orange, and finally to golden-yellow as sodium
concentration $x$ increases from 0.3 to unity. Proper accounts for both the
interband and the intraband contributions to the optical response are found to
be very important for a detailed understanding of the coloration mechanism in
this system.

35
The categories of this paper are: mtrl-th
28
The categories of this paper are: alg-geom
0
The categories of this paper are: hep-ph
1
The categories of this paper are: cs
15
The categories of this paper are: nucl-ex
5
The categories of this paper are: gr-qc
37
The categories of this paper are: supr-con
10
The categories of this paper are: q-bio
8
The categories of this paper are: hep-ex
14
The categories of this paper are: math-ph
36
The categories of this paper are: cmp-lg
34
The categories of this paper are: chem-ph
6
The categories of this paper are: astro-ph
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
31
The categories of this paper are: ao-sci