Sometimes one can improve the theories in the sense of discovering a quicker, more efficient way of doing a given calculation.
describing his discoveries in Density functional theory, December 29, 1995, in an interview with István Hargittai, Magdolna Hargittai (2000). Candid science: conversations with famous chemists, Volume 1 of Candid science. Imperial College Press. p. 180. ISBN 1860942288.