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Manuscript Title: Electron diffraction simulation on Micro-VAX II computers with the aid of an array processor.
Authors: G.Y. Fan
Program title: MIDS
Catalogue identifier: ABLK_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 59(1990)429
Programming language: Fortran.
Computer: MICRO-VAX II.
Operating system: MICRO-VMS.
RAM: 800K words
Word size: 32
Keywords: Solid state physics, Surface, Electron diffraction, Convergent beam, Electron microscope, Inter-atomic structure, Wave function, Phase grating, Elastic scattering, Multislice method.
Classification: 7.2.

Nature of problem:
To simulate the 2-d convergent beam electron diffraction(CBED) patterns produced by focusing a coherent beam, such as that from a field emission gun, on a crystal or defective sample for structure study at the atomic level. The simulation helps researchers to understand various electron diffraction phenomena observed experimentally and to predict new ones.

Solution method:
This program is a numerical implementation of the multislice approach to electron-specimen interaction simulation. The electron wave function at the exit surface of the specimen of thickness H is the result of repetetive transmission of the electron wave through N successive slices, each of thickness Delta z, where H = N Delta z. In each slice the electron-specimen interaction is considered to take place in one plane, which is taken to be the entrance surface of each slice and the electron wave then propagates a distance Delta z, represented by Fresnel diffraction in vacuum. The effect of electron-specimen interaction on the incident wave is represented by a phase change of the incident electron wave function, with absorption and back scattering therefore ignored. Effectively, the specimen is treated as a set of 2-d phase gratings, which multiply the incident electron wave function. Multiple scattering is included.

Restrictions:
Up to 10 distinct phase gratings can be arranged in any order to form a sequence containing up to 10,000 slices. Slices are assumed to have uniform thickness of arbitrary dimension. Practically, however, the slice thickness should be no larger than the crystal unit cell dimension in the direction of electron propagation to avoid error. Up to 300 symmetry operations and 1,500 input atoms of 20 different kinds can be used to construct a superlattice cell. Due to the use of the fast Fourier transform (FFT) technique, the size of any complex data array must be an integral power of 2, e.g., 64*64, 128*256, etc., to a maximum of 256*256.

Running time:
Depending on the complexity of the sample structure and number of beams used, a simulation session can take a few seconds to 30 minutes. For a simulation involving 64*64 beams, the time is 0.1 second per slice if no disk I/O is performed.