COMPUTATION OF VIBRATIONAL FREQUENCIES FOR ADSORBATES ON SURFACES

A general strategy for computing the vibrational frequencies of adsorb ates on surfaces is presented. Rather than use numerical fits to the s urface energy, we advocate diagonalizing a mass weighted energy second -derivative matrix. The computation of energy derivatives has several advantages: It facilitates a more accurate treatment of the adsorbate/ substrate mass ratios. The normal modes of the surface vibrations are given automatically in the diagonalization step. Finally, a major adva ntage of using energy derivatives is that they provide estimates of th e range of the adsorbate/substrate interactions which can be used to e nsure that the frequency values are converged. We illustrate the strat egy for cluster calculations which simulate the H/Al(100), O/Al(111), H2O/Al(111), and Al/Si(111) surfaces. (C) 1997 John Wiley & Sons,Inc.