Perturbations describe an interaction between two states. To add
a perturbation, right-click on the manifold in which the perturbation
is required, and select "Add new..."). There can be more than one
perturbation between any pair of states as there can be more than one
type of interaction that connects states.
These settings are common
to all perturbations; more specific settings are included in the
sections for linear, symmetric tops and asymmetric top molecules.
Settings
Nucleus
Index, starting from 1, of nuclear spin involved in
perturbation; 0 (default) for those not involving a nuclear spin.