"IR Spectra of Phosphate Ions in Aqueous Solution: M Klähn, G Mathias, C Kötting, M Nonella, J Schlitter, K Gerwert, and P Tavan
Predictions of a DFT/MM Approach Compared with Observations"
J. Phys. Chem. A 108, 6186-6194 (2004).
Due to the progress of density functional theory (DFT)
accurate computations of vibrational spectra of isolated molecules
have become a standard task in computational chemistry.
This is not yet the case for solution spectra.
To contribute to the exploration of corresponding computational
procedures, here we suggest a more efficient variant of the
so-called instantaneous normal mode analysis (INMA).
This variant applies conventional molecular dynamics (MD) simulations,
which are based on non-polarizable molecular mechanics (MM)
force fields, to the rapid generation of a large ensemble of
different solvation shells for a solute molecule. Short hybrid simulations,
in which the solute is treated by DFT and the aqueous solvent by MM,
start from snapshots of the MM solute-solvent MD trajectory and
yield a set of statistically independent hydration shells
partially adjusted to the DFT/MM force field.
Within INMA, these shells are kept fixed at their 300 K structures,
line spectra are calculated from the DFT/MM Hessians of the solute,
and its inhomogeneously broadened solution spectra are derived by second order statistics.
As our test application we have selected the phosphate ions HPO4(2-) and
H2PO4(-) because sizable solvation effects are expected for the
IR spectra of these strongly polarizable ions. The widths, intensities, and spectral
positions of the calculated bands are compared with experimental IR spectra recorded by
us for the purpose of checking the computational procedures.
These comparisons provide insights into the merits and limitations of
the available DFT/MM approach to the prediction of IR spectra in condensed phase.
BMO authors (in alphabetic order):
QM/MM hybrid descriptions of solutes in complex solvents