AtomicDensityMatrix`

AllowedCouplings

is an option for WignerEckart and related functions that specifies matrix elements of operators that are allowed to be nonzero.

Details and Options

Matrix elements are specified by crit₁crit₂, where crit₁ and crit₂ are state-selection criteria for the two states coupled by the matrix element. (crit₁ and crit₂ can each designate multiple states.)
The order of crit₁ and crit₂ doesn't matter — for off-diagonal matrix elements, both conjugate matrix elements are always included.
State-selection criteria are specified as for StatePosition: Boolean expressions involving the state parameters, such as StateLabel2&&F1. As a shorthand, if all states with a particular label are to be selected, just the label itself can be specified.

By default, the dipole operator can connect any pair of states of opposite parity:

We can artificially set certain matrix elements to zero by restricting the AllowedCouplings:

Restrict magnetic dipole coupling to act only between two fine-structure states, and not within them: