Evaluation of Spartan semi-empirical molecular modeling software for calculations of molecules on surfaces: CO adsorption on Ni(111)

This paper reports the use of the PM3(tm) semi-empirical method in the Spar tan molecular modeling software to optimize geometries and calculate vibrat ional frequencies for increasingly complex transition metal- and carbon mon oxide (CO)-containing systems, culminating in calculations of CO adsorbed o n a Ni(111) surface. Mononuclear and dinuclear transition metal carbonyl mo lecular species were used to establish the level of accuracy that could be expected for vibrational frequencies to provide a context for the results f rom the adsorbed molecule calculations. One to four CO molecules adsorbed o n the (111) face of a 22-atom-nickel crystal were then modeled, and the acc uracy of the adsorption geometry and vibrational frequency was evaluated. T he calculated CO stretching vibrational frequencies were within 8% larger t han the gas phase experimental values fur the molecular species and were ap proximately 10% larger than the range of experimental values for CO on the nickel surface. The geometry optimization predicted that the CO molecules o n the Ni(111) surface occupy three-fold hollow sites with no preference for sites over Ni atoms, in agreement with recent structural data and other th eoretical calculations. The software was less successful in calculating the CO bond angle to the surface and the distance of the CO molecules from the surface, but the calculation did produce a reasonable distance between CO molecules on the surface. In general, the PM3(tm) method in Spartan shows p romise for predicting adsorption sites and vibrational frequencies of molec ules on metal surfaces. (C) 1001 Elsevier Science Ltd. All rights reserved.