“formal_charge” selects atoms based on their assigned formal charges.
formal_charge > value
formal_charge < value
(formal_charge = value)
fc. > value
fc. < value
(fc. = value)
indicate formal_charge > 0 indicate fc. < 0 indicate (fc. = 1)
Due to parsing limitations, explicit parentheses must be used around expressions with equals signs “=” in order to distinguish them from assignment of a keyword parameter to a command.