brainchild vs opus

brainchild

noun
  • A creation, original idea, or innovation, usually used to indicate the originators 

verb
  • To think up; to come up with. 

opus

noun
  • A work of music or set of works with a specified rank in an ordering of a composer's complete published works. 

  • A work, especially of art. 

How often have the words brainchild and opus occurred in a corpus of books? (source: Google Ngram Viewer )