Martin Karplus of the Université de Strasbourg and Harvard University, Michael Levitt of Stanford University, and Arieh Warshel of the University of Southern California shared the prize for laying the foundation "for the powerful programs that are used to understand and predict chemical processes," the academy said in a statement. Here's more:
The strength of classical physics was that calculations were simple and could be used to model really large molecules. Its weakness, it offered no way to simulate chemical reactions. For that purpose, chemists instead had to use quantum physics. But such calculations required enormous computing power and could therefore only be carried out for small molecules. This year’s Nobel Laureates in chemistry took the best from both worlds and devised methods that use both classical and quantum physics.
The academy cites drug delivery as an example: "In simulations of how a drug couples to its target protein in the body, the computer performs quantum theoretical calculations on those atoms in the target protein that interact with the drug. The rest of the large protein is simulated using less demanding classical physics."
For more coverage of the 2013 Nobel Prizes, go here and here.
