Test of structural models for the Si{111} 2 × 1 surface

New low-energy electron diffraction (LEED) experiments and new calculations on Si{111} 2 ×{} 1 surfaces have produced the following results: (1) LEED intensity data from different surfaces cleaved in different directions in different experiments are reproduced well---thus, if two or more different 2 ×{} 1 superstructures occur simultaneously on cleaved surfaces, their propertions are always the same; (2) LEED intensity calculations of different authors with different computer programs are reproducible---thus, confidence in the reliability of the calculations is confirmed; (3) the buckling model, the π-bonded chain model, the Seiwatz model, and the Chadi molecular model all fail the LEED test---thus, the Si{111} 2 ×{} 1 structural problem is still unsolved. A complete test of these models requires consideration of distortions in deeper atomic layers, so far unknown.