Quantum Monte Carlo, time-dependent density functional theory, and density functional theory calculations of diamondoid excitation energies and Stokes shifts

Details Quantum Monte Carlo, time-dependent density functional theory, and density functional theory calculations of diamondoid excitation energies and Stokes shifts Quantum Monte Carlo, time-dependent density functional theory, and density functional theory calculations of diamondoid excitation energies and Stokes shifts

2011-05-12

arxiv.org/abs/1105.2520v1

Phys. Rev. B 84, 245315 (2011)

Physics

Atomic and Molecular Clusters

29 pages,6 figures,3 tables

Scientific paper

10.1103/PhysRevB.84.245315

We have computed the absorption and emission energies and hence Stokes shifts of small diamondoids as a function of size using different theoretical approaches, including density functional theory and quantum Monte Carlo (QMC) calculations. The absorption spectra of these molecules were also investigated by time-dependent density functional theory (TD-DFT) and compared with experiment. We have analyzed the structural distortion and formation of a self-trapped exciton in the excited state, and we have studied the effects of these on the Stokes shift as a function of size. Compared to recent experiments, QMC overestimates the excitation energies by about 0.8(1) eV on average. Benefiting from a cancellation of errors, the optical gaps obtained in DFT calculations with the B3LYP functional are in better agreement with experiment. It is also shown that TD-B3LYP calculations can reproduce most of the features found in the experimental spectra. According to our calculations, the structures of diamondoids in the excited state show a distortion which is hardly noticeable compared to that found for methane. As the number of diamond cages is increased, the distortion mechanism abruptly changes character. We have shown that the Stokes shift is size-dependent and decreases with the number of diamond cages. The rate of decrease in the Stokes shift is on average 0.1 eV per cage for small diamondoids.

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