 These empirical methods were replaced in the 1960s by semi-empirical methods such as CNDO.In the early 1970s, efficient "ab initio" computer programs such as ATMOL, Gaussian, IBMOL, and POLYAYTOM, began to be used to speed "ab initio" calculations of molecular orbitals.Of these four programs, only Gaussian, now vastly expanded, is still in use, but many other programs are now in use.