The excited-state intramolecular H-atom transfer reactions of hypocrellins B and A are compared with those of calphostin C. On the basis of the results of transient absorption measurements and ab initio quantum mechanical calculations, it is concluded that large-amplitude conformational changes are coupled to the H-atom transfer in calphostin C, just as they are in hypocrellins A and B. The calculations on this very large molecule with a very complex ground electronic state potential energy surface were made possible by the use of highly scalable electronic structure theory codes on large parallel computers.