I can't figure out how to call a function with a trigger, keeps saying function doesn't exist
Before creating trigger, you need to create a trigger function, which has to be (quoting the manual):


  A user-supplied function that is declared as taking no arguments and returning type trigger


Like: 

CREATE OR REPLACE FUNCTION calculate_t_f()
  RETURNS trigger AS
$$
BEGIN
...
-- do something
-- maybe with with calculate(char)...
...
END
$$ LANGUAGE plpgsql;


Then you can create your trigger with that trigger function (not your original function):

CREATE TRIGGER trigger_calculate
AFTER INSERT ON patient_notitie
EXECUTE PROCEDURE calculate_t_f();