Ab initio determination of crystal structures based on a maximum entropy formalism has produced a wealth of papers debating the utility of the method, but very few actual determinations. On the other hand, a maximum entropy formalism aiming at the production of a ``maximally non-committal'' map is an almost standard method in fields of science like radioastronomy, but a rare exception in both X-ray crystallography and electron microscopy (or crystallography).
The calculation of a maximum entropy map when an atomic model can be built in a conventional
F
oexp(i) (or F
oexp(i
)) synthesis, is clearly a waste of computer time.
But when the map is the end product
(low resolution electron or potential density maps, Patterson functions), the calculation of the MAXENT map can be more than
useful (see section 3 for examples).
The programme MAXENT will read a CCP4 map file and will calculate a map that maximises the Shannon-Jaynes (or configurational) entropy subject to the constraint that the final map is consistent with the observed data. Section 2 describes the essentials of the method, section 3 shows results from model calculations and section 4 discusses the use (or misuse) of the programme.