Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Electron Correlation in the Lanthanides: $4f^2$ spectrum of Ce$^{2+}$   Atoms and ions of Lanthanides have multiple opens shells along with an open
$4f^k$ subshell. This paper studies the effect of electron correlation in such
systems and how wave functions can be determined for the accurate prediction of
atomic properties in the case of Ce$^{2+}$ where $k=2$, using the
multireference single- and double-excitation method. An efficient higher-order
method is recommended for more reliable results.

4
The categories of this paper are: cond-mat
9
The categories of this paper are: nlin
27
The categories of this paper are: comp-gas
31
The categories of this paper are: ao-sci
11
The categories of this paper are: hep-lat
8
The categories of this paper are: hep-ex
7
The categories of this paper are: hep-th
28
The categories of this paper are: alg-geom
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
35
The categories of this paper are: mtrl-th
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
22
The categories of this paper are: chao-dyn
19
The categories of this paper are: eess