Comparison of basis set effects and the performance of ab initio and DFT methods for probing equilibrium fluctuations |
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Authors: | Walker Ross C Mercer Ian P Gould Ian R Klug David R |
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Affiliation: | Biophysics and Biological Chemistry Group, Department of Chemistry, Imperial College London, London, UK. ross@rosswalker.co.uk |
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Abstract: | The electronic absorption and emission spectra of large molecules reflect the extent and timescale of electron-vibration coupling and therefore the extent and timescale of relaxation/reorganization in response to a perturbation. In this paper, we present a comparison of the calculated absorption and emission spectra of NADH in liver alcohol dehydrogenase (LADH), using quantum mechanical/molecular mechanical methods, in which we vary the QM component. Specifically, we have looked at the influence of basis set (STO-3G, 3-21G*, 6-31G*, CC-pVDZ, and 6-311G**), as well as the influence of applying the DFT TD-B3LYP and ab initio TD-HF and CIS methods to the calculation of absorption/emission spectra and the reorganization energy (Stokes shift). The ab initio TD-HF and CIS methods reproduce the experimentally determined Stokes shift and spectral profiles to a high level of agreement, while the TD-B3LYP method significantly underestimates the Stokes shift, by 45%. We comment on the origin of this problem and suggest that ab initio methods may be naturally more suited to predicting molecular behavior away from equilibrium geometries. |
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Keywords: | LADH TD‐HF TD‐DFT CIS basis set fluctuation dissipation theory |
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