Fast and Accurate Quantum Chemical Modeling of Infrared Spectra of Condensed-Phase Systems

An efficient approach for an accurate quantum mechanical (QM) modeling
of infrared (IR) spectra of condensed-phase systems is described.
An ensemble of energetically low-lying cluster structures of a solute
molecule surrounded by an explicit shell of solvent molecules is efficiently
generated at the semiempirical tight-binding QM level and then reoptimized
at the density functional theory level of theory. The IR spectrum
of the solvated molecule is obtained as a thermodynamic average of
harmonically computed QM spectra for all significantly populated cluster
structures. The accuracy of such simulations in comparison to experimental
data for some organic compounds and their solutions is shown to be
the same or even better than the corresponding QM computations of
the gas-phase IR spectrum for the isolated molecule.