Decomposing total IR spectra of aqueous systems into solute and solvent contributions: A computational approach using maximally localized Wannier orbitals

Abstract

The theoretical principles underpinning the calculation of infrared spectra for condensed-phase systems in the context of ab initio molecular dynamics have been recently developed in literature. At present, most ab initio molecular dynamics calculations are restricted to relatively small systems and short simulation times. In this paper we devise a method that allows well-converged results for infrared spectra from ab initio molecular dynamics simulations using small systems and short trajectories characteristic of simulations typically performed in practice. We demonstrate the utility of our approach by computing the imaginary part of the dielectric constant epsilon"(omega) for H2O and D2O in solid and liquid phases and show that it compares well with experimental data. We further demonstrate that maximally localized Wannier orbitals can be used to separate the individual contributions of different molecular species to the linear spectrum of complex systems. The new spectral decomposition method is shown to be useful in present-day ab initio molecular dynamics calculations to compute the magnitude of the "continuous absorption" generated by excess protons in aqueous solutions with good accuracy even when other species present in the solutions absorb strongly in the same frequency window.

Real-World Predictions from Ab Initio Molecular Dynamics Simulations

“…Another difference in solution could be an enhanced inertia (giving rise to frequency red-shifts) due to the fictitious electron mass used in the Car-Parrinello molecular dynamics scheme. As also shown by Iftimie and Tuckerman, the fictitious electron mass can contribute to the underestimation of the frequencies, up to 40-50 cm À1 [72]. If the well-known frequency red-shifts due to the use of the BLYP functional are kept in mind, this leads to an increased underestimation of the frequency positions [39].…”

“…3. In 2005, Iftimie and Tuckerman devised a method that allows well-converged results for IR spectra from small AIMD systems and short trajectories [72]. The frequency-(n)-dependent Beer-Lambert absorptivity coefficient a(n) is given as…”

Can far-IR action spectroscopy combined with BOMD simulations be conformation selective?

“…For the prefactor in eqn (1), we have taken into account an empirical quantum correction factor (multiplying the classical line shape) of the form bho/(1 À exp (Àbho)), which was shown by us and others to give accurate results on calculated IR intensities. [43][44][45] For more detailed discussions on quantum corrections, see for instance ref. 46-49. The main advantages of the molecular dynamics (MD) approach in eqn (1) for the calculation of infrared spectra (also called ''dynamical spectra'' in the remainder of the text) are discussed in detail in our review 50 and are briefly listed as follows:…”

Theoretical Methods for Vibrational Spectroscopy and Collision Induced Dissociation in the Gas Phase

“…The calculation in (5) is done in the absence of an applied external field. For the prefactor in (5), we have taken into account an empirical quantum correction factor (multiplying the classical line shape) of the form βℏω= 1 À exp Àβℏω ð Þ ð Þ , which was shown by us and others to give accurate results on calculated IR intensities [30][31][32]. For more detailed discussions on quantum corrections, see for instance [33][34][35][36].…”