On the Use of Locally Dense Basis Sets in the Calculation of EPR Hyperfine Couplings: A Study on Model Systems for Bio-Inorganic Fe and Co Complexes
Birgitte O. Milhoj, Erik D. Hedegard and Stephan P. A. Sauer
Affiliation: Department of Chemistry, University of Copenhagen, DK-2100 Copenhagen O, Denmark.
The usage of locally dense basis sets in the calculation of Electron Paramagnetic Resonance (EPR) hyperfine
coupling constants is investigated at the level of Density Functional Theory (DFT) for two model systems of biologically
important transition metal complexes: One for the active site in the compound 0 intermediate of cytochrome P450cam,
[Fe(OOH)(SH)(en)2]+, and one for the active site in coenzyme B12, [Co(NH3)(CN)(en)2]+. The Fermi contact, spin-dipolar
and second order paramagnetic spin-orbit coupling contributions to the hyperfine coupling tensors of the metal and the
ligating ethylenediamine N atoms are calculated, and their dependence on the basis set for the remaining atoms are investigated.
Core property basis sets are employed for the metals (aug-cc-pVTZ-Juc) and their equatorially coordinating N
atoms (aug-cc-pVTZ-J or 6-31G-Juc analogues to the Pople style basis sets used for the remaining atoms), while smaller
correlation-consistent or Pople style basis sets are used for the remaining, so-called “non-coupled'', atoms. Most of the investigated
basis set combinations are found to give results which differ by less than 1% from the results obtained with
core property basis sets on all atoms. We find thus for the cytochrome model system that using the small 6-31G(d) basis
set on the non-coupled atoms together with core property basis sets on the Fe and N atoms gives essentially converged results.
It is found to be mostly the second order paramagnetic spin-orbit interaction that demands the use of larger basis sets
on the non-coupled atoms. If, however, an error of less than 0.5MHz is sufficient any basis set can be used for the noncoupled
atoms. For the cobalt containing model system the 6-31G(2d) basis set generally gives results within 1% of the
Keywords: Aug-cc-pVTZ-J, cytochrome P450, EPR hyperfine coupling constant, locally dense basis set, vitamin B12.
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