SIMILARITY OF ATOMS IN MOLECULES

Similarity of atoms in molecules is quantitatively assessed with a mea sure that employs electron densities within respective atomic basins. This atomic similarity measure does not rely on arbitrary assumptions concerning basis functions or ''atomic orbitals'', is relatively inexp ensive to compute, and has straightforward interpretation. Inspection of similarities between pairs of carbon, hydrogen, and fluorine atoms in the CH4, CH3F, CH2F2, CHF3, CF4, C2H2, C2H4, and C2H6, molecules, c alculated at the MP2/6-311G* level of theory, reveals that the atomic similarity is greatly reduced by a change in the number or the charac ter of ligands (i.e. the atoms with nuclei linked through bond paths t o the nucleus of the atom in question). On the other hand, atoms with formally identical (i.e. having the same nuclei and numbers of ligands ) ligands resemble each other to a large degree, with the similarity i ndices greater than 0.95 for hydrogens and 0.99 for non-hydrogens.