Vibrational Spectra of Highly Anharmonic Water Clusters: Molecular Dynamics and Harmonic Analysis Revisited with Constrained Nuclear-Electronic Orbital Methods

Vibrational spectroscopy is widely used to gain insights into structural and dynamic properties of chemical, biological, and materials systems. Thus, an efficient and accurate method to simulate vibrational spectra is desired. In this paper, we justify and employ a microcanonical molecular simulation scheme to calculate the vibrational spectra of three challenging water clusters: the neutral water dimer (H4O2), the protonated water trimer (H7O3+), and the protonated water tetramer (H9O4+). We find that with the accurate description of quantum nuclear delocalization effects through the constrained nuclear-electronic orbital framework, including vibrational mode coupling effects through molecular dynamics simulations can additionally improve the vibrational spectrum calculations. In contrast, without the quantum nuclear delocalization picture, conventional ab initio molecular dynamics may even lead to less accurate results than harmonic analysis.

Analytic intermolecular potential energy surface and first-principles prediction of the rotational profiles for a symmetric top ion-atom complex: A case study of H3O+-Ar

We presented the first three-dimensional (3D) ab initio intermolecular potential energy surface (PES) for the H₃O⁺-Ar complex. The electronic structure computations were carried out at the explicitly correlated coupled cluster theory-F12 with an augmented correlation-consistent triple zeta basis set. Analytic 3D PES was obtained by least-squares fitting the multi-dimensional Morse/Long-Range (mdMLR) potential model to interaction energies, where the mdMLR function form was applied to the nonlinear ion-atom case for the first time. The 3D PES fitting to 1708 points has root-mean-square deviations of 0.19 cm^-1 with only 108 parameters for interaction energies less than 500 cm^-1. With the 3D PES of the H₃O⁺-Ar complex, we employed the combined radial discrete variable representation/angular finite basis representation method and Lanczos algorithm to calculate rovibrational energy levels. The rotational profiles of the O-H anti-stretching vibrational bands of v₃ ⁺(S)←0⁺ and v₃ ^-(A)←0^- for the H₃O⁺-Ar complex were predicted and were in good agreement with the experimental results.

Ab Initio Investigation of Nonlinear Mode Coupling in C60

Strong light fields may be used to control molecular structure, thus providing a route to new, light-induced phases of matter. In this context, we present an ab initio molecular dynamics investigation of nonlinear mode coupling in C₆₀ and show that, under suitable conditions, resonant infrared excitation induces significant structural changes in the system. Surprisingly, exciting the highest-frequency infrared mode at field strengths employed in a recent experiment [Nature 2016, 530, 461-464], we observe no significant structural change. However, when targeting the first two infrared modes using stronger fields, all Raman modes gain energy through nonlinear coupling to the infrared modes, leading to large-amplitude vibrational excitations. We find a strong response of the H_g(5) mode, which is symmetric with respect to the equilibrium structure. For sufficiently strong field, it is of the same order as the infrared modes' excitation. This encourages further investigations into the light-induced superconducting properties of alkali-doped fullerenes.

Tuning the vibrational coupling of H3O+ by changing its solvation environment

This study demonstrates how the intermode coupling in the hydronium ion (H₃O⁺) is modulated by the composition of the first solvation shell.

Coordination structure and charge transfer in microsolvated transition metal hydroxide clusters [MOH]+(H2O)1–4

Infrared vibrational predissociation spectra of transition metal hydroxide clusters, [MOH]⁺(H₂O)_1–4·D₂ with M = Mn, Fe, Co, Ni, Cu, and Zn, are presented and analyzed, showing solvent driven changes in coordination and charge transfer.

Deciphering the infrared spectrum of the protonated water pentamer and the hybrid Eigen-Zundel cation

Traditionally, infrared band assignment for the protonated water clusters, such as H(+)(H2O)5, is based on their lowest energy isomer. Recent experiments extend the observation spectral window to lower frequencies, for which such assignment appears to be inadequate. Because this hydrogen-bonded system is highly anharmonic, harmonic spectral calculations are insufficient for reliable interpretation. Consequently, we have calculated the IR spectrum of several isomers of the protonated water pentamer using an inherently anharmonic methodology, utilizing dipole and velocity autocorrelation functions computed from ab initio molecular dynamic trajectories. While the spectrum of H(+)(H2O)5 is universally assumed to represent the branched Eigen isomer, we find a better agreement for a mixture of a ring and linear isomers. The first has an Eigen core and contributes at high frequencies, whereas the latter accounts for all prominent low-frequency bands. Interestingly, its core is neither a classical Eigen nor a Zundel cation, but rather has hybrid geometry. Such an isomer may play a role in proton conductance along short proton wires.