Configurations are stored in the database using an unambiguous notation which should be familiar to most users. A configuration consists of a space-delimited list of sub-shells in standard order each having the form, , where is the sub-shell (standard order: 1s, 2s, 2p, 3s, ...) and is the occupation number. Note that the shorthand notation of omitting when unity is not used, e.g. 2s1 not 2s. Configuration strings obey the rules:
Using a list of occupation numbers as the configuration label was considered and ultimately rejected due to the impracticality of storing Rydberg levels. Consider the configuration, 1s 200p; whereas only 13 characters are needed to store this configuration in the form described above, nearly 40000 characters are required if using a list of occupation numbers.
To get the number of electrons of a configuration takes two steps; first you
need to calculate the number of electrons in the core and then add up the
occupation numbers of the visible sub-shells.
To get the number of electrons in the core, , take the principal
quantum number, , and the orbital angular momentum, of the first
open sub-shell and apply the following expression: