Editing Electron configuration (section)

== Open and closed shells ==
{{About|the concept in physics|the software|Open Shell|section=TRUE}}
In the context of [[atomic orbital]]s, an '''open shell''' is a [[valence shell]] which is not completely filled with [[electron]]s or that has not given all of its valence electrons through [[chemical bond]]s with other [[atom]]s or [[molecule]]s during a [[chemical reaction]]. Conversely a '''closed shell''' is obtained with a completely filled valence shell. This configuration is very [[Stable nuclide|stable]].<ref>{{cite web|url=http://www.newi.ac.uk/buckleyc/periodic.htm|title=Periodic table|access-date=2007-11-01|archive-url=https://web.archive.org/web/20071103074338/http://www.newi.ac.uk/buckleyc/periodic.htm|archive-date=2007-11-03|url-status=dead}}</ref>

For molecules, "open shell" signifies that there are [[Unpaired electron|unpaired electrons]]. In [[molecular orbital]] theory, this leads to molecular orbitals that are singly occupied. In [[computational chemistry]] implementations of molecular orbital theory, open-shell molecules have to be handled by either the [[restricted open-shell Hartree–Fock]] method or the [[unrestricted Hartree–Fock]] method. Conversely a closed-shell configuration corresponds to a state where all [[molecular orbital]]s are either doubly occupied or empty (a [[diradical|singlet state]]).<ref>{{cite book|chapter-url=http://www.semichem.com/ampacmanual/ci.html |url=http://www.semichem.com/ampacmanual/ |publisher=Semichem, Inc. |chapter=Chapter 11. Configuration Interaction|title=AMPAC™ 10 User Guide}}</ref>  Open shell molecules are more difficult to study computationally.<ref>{{cite web|url=http://iopenshell.usc.edu/|title=Laboratory for Theoretical Studies of Electronic Structure and Spectroscopy of Open-Shell and Electronically Excited Species – iOpenShell|website=iopenshell.usc.edu}}</ref>