Title ofprogram: XFIT Nature ofphysical problem A structure hypothesis relative to a powder, a polycrystalline or Catalogue number: ACKY a glassy material can be tested by comparison between the experimental X-ray powder pattern and a calculated one. Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland (see application form in this issue) Method ofsolution XFIT consists of three programs: PROVA2, GSFIT2 and Computer: CYBER 76; Installation: CINECA. Casalecchio di MINUITS. PROVA2 generates the calculated patterns by Reno, Bologna, Italy evaluation of one surface integral for each calculated intensity; GSFIT2 converts into analytical form the dependence of the Operating systems: NOS/BE 1.3 calculated patterns on the structural parameters; MINUITS (by the supported FCN subroutine) optimizes the structural Programming language used: FORTRAN CDC parameters to achieve the best fit to the experimental pattern. GSFIT2 and MINUITS may be called as subroutines by High speed storage required: 56 Kwords PROVA2 or run separately as independent programs. No. of bits in a word: 60 Restrictions on the complexity of the problem The dimensioning of the arrays allows up to 100 intensity Peripherals used: card reader, line printer, magnetic disk values and 6 structural parameters to be handled No. of lines in combinedprogram and test deck: 4250 Typical running time Depending on the complexity of the proposed structural mode; CPC Library subprograms used: ABCH (GSFIT) and ACWH the one presented in the test run requires 1.75 s per intensity (MINUIT) value.