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Hello,

I've been performing frequency calculations on oxygen-oxygen stretches. I've performed frequency calculations of O2 on Pt(111) to compare with Eichler and Hafner's work PRL V79 p.4481 (1997), which compare very favorably to experiment. I've used a slightly different calculation setup -

3 frozen substrate layers
1/9 ML O2 coverage
PAW potentials
5x5x1 Monkhorst Pack k-point mesh
ENCUT = 400
POTIM = 0.04

For both the magnetic (top-bridge-top) and nonmagnetic (top-fcc-bridge) states, my adsorption energies and geometries compare very well to Eichler and Hafner's values. And the O-O stretching frequency for the magnetic state is very close (within 3 cm-1). However, the O-O stretch for the nonmagnetic state is 826cm-1, compared to 690 cm-1 calculated by Eichler/Hafner. The optimized geometry was converged with forces on the oxygen atoms are < 0.005 eV/A. Can anyone offer suggestions as to what is going on?

Thanks,
Matt

How do your values compare to experiment? That's always the perfect test ...
Was the coverage the same as in Hafner/Eichler's work?
Have you used decent pseudopotentials for oxygen? O-O bond comes with r=124pm, so O_s is too bad in forces, but might give reasonable structures ...

Hth
Alex

That's the thing. Structures and energies agree very well. It's just the frequency. The frequencies calculated by Eichler and Hafner compare very favorably to experiment. The question is: why would the frequency be so far off when everything else is correct?