Vibrational Spectroscopy in Solution through Perturbative ab Initio Molecular Dynamics Simulations.

We have developed a method that allows computing the vibrational spectra at a high quantum mechanical level for molecules in solution or other complex systems. The method is based on the use of configurational samplings from combined QM/MM molecular dynamics simulations and the use of perturbation theory to calculate accurate molecular properties. Such calculations provide in addition accurate free energy gradient vectors and Hessian matrices and thus open the door for the characterization of stationary points in free energy landscapes and the study of chemical reaction mechanisms in large disordered systems. The vibrational spectrum of the water molecule in liquid water has been computed as a test case. It has been obtained using a weighted average of instantaneous signals assuming the instantaneous normal modes approach. Vibrational frequencies are also computed by diagonalizing the Hessian of the free energy surface. Comparison is made with experimental data and with calculations using the Fourier transform of the time autocorrelation function of the dipole moment. The discussion emphasizes the advantages of the developed methodology compared to other techniques in terms of the accuracy/computational cost ratio.