Chiral crystal surfaces lack mirror or glide plane symmetry. Nevertheless, some chiral surfaces deviate more significantly from an achiral configuration, and thus possess greater enantioselective potential, than others. We describe a procedure to calculate chiral indices, I-C (in Angstrom), of any two-dimensional (2D) periodic atomic surface based on atomic displacements from ideal mirror or glide plane symmetry. We define a 2D unit cell parallel to the surface, identify coordinates of atoms associated with that surface unit cell, and employ minimization procedures to determine the positions and orientations of best-fit pseudo-mirror and pseudo-glide plane operators perpendicular to that surface. Achiral surfaces invariably have I-C = 0, but we find that surfaces of intrinsically chiral crystals [e.g., quartz (101)] may also display I-C = 0, depending on the surface atoms selected. Of 14 surfaces modeled, I-C is greatest for chiral faces of achiral crystals: the (2 14) scalenohedral faces of calcite (I-C = 2.60 Angstrom), the (110) faces of diopside (I-C = 1.54 Angstrom), and the (6 4 3) faces of FCC metals such as copper and platinum (I-C = 1.29 Angstrom). (C) 2004 Elsevier B.V. All rights reserved.