Starting from JPA 2.1 custom functions can be called like

Author a = em.createQuery("SELECT a FROM Author a WHERE a.id = function('calculate', 1, 2)", Author.class).getSingleResult();

But this fact is missing in the documentation:
https://docs.jboss.org/hibernate/orm/current/userguide/html_single/Hibernate_User_Guide.html#hql-non-standard-functions

Please add a chapter that describes this JPQL feature.