  The influence of the partial charge distribution obtained from quantum
mechanics of the solute 1-methyl-6-oxyquinolinium betaine in the ground- and
first excited state on the time-dependent Stokes shift is studied via molecular
dynamics computer simulation. Furthermore, the effect of the employed solvent
model - here the non-polarizable SPC, TIP4P and TIP4P/2005 and the polarizable
SWM4 water model - on the solvation dynamics of the system is investigated. The
use of different functionals and calculation methods influences the partial
charge distribution and the magnitude of the dipole moment of the solute, but
not the orientation of the dipole moment. Simulations based on the calculated
charge distributions show nearly the same relaxation behavior. Approximating
the whole solute molecule by a dipole results in the same relaxation behavior,
but lower solvation energies, indicating that the time scale of the Stokes
shift does not depend on peculiarities of the solute. However, the SPC and
TIP4P water models show too fast dynamics which can be ascribed to a too large
diffusion coefficient and too low viscosity. The calculated diffusion
coefficient and viscosity for the SWM4 and TIP4P/2005 model coincide well with
experimental values and the corresponding relaxation behavior is comparable to
experimental values. Furthermore we found that for a quantitative description
of the Stokes shift of the applied system at least two solvation shells around
the solute have to be taken into account.
