There is generally no article when a singular proper noun is used as a possessive:
- Beethoven's Fifth Symphany. John's car. Einstein's theory of
Plural proper nouns used as possessives generally do use an article:
- The Beethovens's dinner was getting cold. The Yankees' locker room.
The Marx Brothers' last film.
We don't use articles with determiners that function as possessives (sometimes called possessive pronouns):
His most famous work.
There are exceptions, fixed expressions, and special ways to create particular meanings--That's the Mary's smile I wanted to see.