“p.” or properties['<property-name>'] in alter, iterate, iterate_state, or alter_state commands can access or set user-defined properties that are atom-level (alter/iterate) or atom-state level (iterate_state/alter_state). atom-state means that these settings are set for a specific atom in a specific state.

# set a property for each atom:
alter_state 1, all, p.xy = x + y
# creates a selection foo that includes only atoms with x+y < 50
select foo, p.xy < 50

In the selection language, only the “p.” notation is supported.

• Selection example on properties page
• Selections