The SCC-DFTB method applied to biological systems: Successes, problems and
extensions

Marcus Elstner, m.elstner@dkfz.de, Theoretical Chemistry, TU Braunschweig,
Hans-Sommer-Straße 10, 38106 Braunschweig, Germany

The approximate Density Functional Theory Method (SCC-DFTB) is derived from DFT
by a second order expansion of the total energy expression with respect to the
charge density [1]. In this talk, we:

1)Evaluate the method with respect to its accuracy describing organic and
  biological molecules in comparison with other semi-empirical methods and
  DFT. This includes reaction energies, geometries and vibrational frequencies
  of small organic molecules and also structural and energetic properties of
  larger biological molecules, like polypeptides and DNA fragments.

2)Discuss limitations/extensions of the second order formalism: inclusion of
  third order terms in the charge density fluctuations allows for a better
  treatment of highly charged systems, which is in particular relevant for the
  calculation of proton affinities. Further, we critically review the treatment
  of the electron-electron repulsion in DFTB and suggest a modification, which
  leads to an improved description of H-bonded systems. An evaluation for
  H-bonded systems and PAs is given.

3)Illustrate cases, where the underlying DFT description leads to a failure of
  SCC-DFTB. The local nature of the available correlation functionals does not
  allow for treatment of VdW systems within DFT. Since this shortcoming is
  inherited by SCC-DFTB, we have included dispersion forces empirically because
  of their crucial role for the stability of protein and DNA structures [2]. The
  local nature of common DF exchange functionals leads to problems in the
  description of ionic and charge transfer excited states.

4)Review the usage of DFTB within hybrid QM/MM [3] methods and the application
  to biological systems.

[1] Elstner et al., Phys. Rev. B 58 (1998) 7260 [2] Elstner et
al.,J. Chem. Phys. 114 (2001) 5149 [3] Cui et al., J. Phys. Chem. B 105 (2001)
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