Quantum Calculations of Intramolecular IR Spectra of Ice Models Using Ab Initio Potential and Dipole Moment Surfaces

Fully Quantum Simulation of Polaritonic Vibrational Spectra of Large Cavity-Molecule System

The formation of molecular vibrational polaritons, arising from the interplay between molecular vibrations and infrared cavity modes, is a quantum phenomenon necessitating accurate quantum dynamical simulations. Here, we introduce the cavity vibrational self-consistent field/virtual state configuration interaction method, enabling quantum simulation of the vibrational spectra of many-molecule systems within the optical cavity. Focusing on the representative (H2O)21 system, we showcase this parameter-free quantum approach's ability to capture both linear and nonlinear vibrational spectral features. Our findings highlight the growing prominence of molecular couplings among OH stretches and bending excited bands with increased light-matter interaction, revealing distinctive nonlinear spectral features induced by vibrational strong coupling.

Predicting the Raman Spectra of Liquid Water with a Monomer-Field Model

The Raman spectrum of liquid water is quite complex, reflecting its strong sensitivity to the local environment of the individual waters. The OH-stretch region of the spectrum, which captures the influence of hydrogen bonding, has only just begun to be unraveled. Here we develop a model for predicting the Raman spectra of the OH-stretch region by considering how local electric fields distort the energy surface of each water monomer. We find that our model is capable of reproducing the bimodal nature of the main peak, with the shoulder at 3250 cm-1 resulting almost entirely from Fermi resonance. Furthermore, we capture the temperature and polarization dependence of the shoulder, which has proven to be difficult to obtain with previous methods, and analyze the origin of this dependence. We expect our model to be generally useful for understanding and predicting how Raman spectra change under different conditions and with different probe reporters beyond water.

Manipulating hydrogen bond dissociation rates and mechanisms in water dimer through vibrational strong coupling

The vibrational strong coupling (VSC) between molecular vibrations and cavity photon modes has recently emerged as a promising tool for influencing chemical reactivities. Despite numerous experimental and theoretical efforts, the underlying mechanism of VSC effects remains elusive. In this study, we combine state-of-art quantum cavity vibrational self-consistent field/configuration interaction theory (cav-VSCF/VCI), quasi-classical trajectory method, along with the quantum-chemical CCSD(T)-level machine learning potential, to simulate the hydrogen bond dissociation dynamics of water dimer under VSC. We observe that manipulating the light-matter coupling strength and cavity frequencies can either inhibit or accelerate the dissociation rate. Furthermore, we discover that the cavity surprisingly modifies the vibrational dissociation channels, with a pathway involving both water fragments in their ground vibrational states becoming the major channel, which is a minor one when the water dimer is outside the cavity. We elucidate the mechanisms behind these effects by investigating the critical role of the optical cavity in modifying the intramolecular and intermolecular coupling patterns. While our work focuses on single water dimer system, it provides direct and statistically significant evidence of VSC effects on molecular reaction dynamics.

Multidimensional Quantum Dynamical Simulation of Infrared Spectra under Polaritonic Vibrational Strong Coupling

Recent experimental and theoretical studies demonstrate that the chemical reactivity of molecules can be modified inside an optical cavity. Here, we provide a theoretical framework for conducting multidimensional quantum simulations of the infrared (IR) spectra for molecules interacting with cavity modes. A single water molecule under polaritonic vibrational strong coupling serves as an illustrative example. Combined with accurate potential energy and dipole moment surfaces, our cavity vibrational self-consistent field/virtual state configuration interaction (cav-VSCF/VCI) approach can predict the IR spectra when the molecule is inside or outside the cavity. The spectral signatures of Rabi splittings and shifts of certain bands are found to be strongly dependent on the frequency and polarization direction of the cavity modes. Analyses of the simulated spectra show that polaritonic vibrational strong coupling can induce unconventional couplings among the molecule's vibrational modes, suggesting that intramolecular vibrational energy transfer can be significantly accelerated by the cavity.

Anharmonic Vibrational Calculations Based on Group-Localized Coordinates: Applications to Internal Water Molecules in Bacteriorhodopsin

An efficient anharmonic vibrational method is developed exploiting the locality of molecular vibration. Vibrational coordinates localized to a group of atoms are employed to divide the potential energy surface (PES) of a system into intra- and inter-group contributions. Then, the vibrational Schrödinger equation is solved based on a PES, in which the inter-group coupling is truncated at the harmonic level while accounting for the intra-group anharmonicity. The method is applied to a pentagonal hydrogen bond network (HBN) composed of internal water molecules and charged residues in a membrane protein, bacteriorhodopsin. The PES is calculated by the quantum mechanics/molecular mechanics (QM/MM) calculation at the level of B3LYP-D3/aug-cc-pVDZ. The infrared (IR) spectrum is computed using a set of coordinates localized to each water molecule and amino acid residue by second-order vibrational quasi-degenerate perturbation theory (VQDPT2). Benchmark calculations show that the proposed method yields the N-D/O-D stretching frequencies with an error of 7 cm-1 at the cost reduced by more than five times. In contrast, the harmonic approximation results in a severe error of 150 cm-1. Furthermore, the size of QM regions is carefully assessed to find that the QM regions should include not only the pentagonal HBN itself but also its HB partners. VQDPT2 calculations starting from transient structures obtained by molecular dynamics simulations have shown that the structural sampling has a significant impact on the calculated IR spectrum. The incorporation of anharmonicity, sufficiently large QM regions, and structural samplings are of essential importance to reproduce the experimental IR spectrum. The computational spectrum paves the way for decoding the IR signal of strong HBNs and helps elucidate their functional roles in biomolecules.

High-Level VSCF/VCI Calculations Decode the Vibrational Spectrum of the Aqueous Proton

The hydrated excess proton is a common species in aqueous chemistry, which complexes with water in a variety of structures. The infrared spectrum of the aqueous proton is particularly sensitive to this array of structures, which manifests as continuous IR absorption from 1000 to 3000 cm^-1 known as the "proton continuum". Because of the extreme breadth of the continuum and strong anharmonicity of the involved vibrational modes, this spectrum has eluded straightforward interpretation and simulation. Using protonated water hexamer clusters from reactive molecular dynamics trajectories, and focusing on their central H⁺(H₂O)₂ structures' spectral contribution, we reproduce the linear IR spectrum of the aqueous proton with a high-level local monomer quantum method and highly accurate many-body potential energy surface. The accuracy of this approach is first verified in the vibrational spectra of the two isomers of the protonated water hexamer in the gas phase. We then apply this approach to 800 H⁺(H₂O)₆ clusters, also written as [H⁺(H₂O)₂](H₂O)₄, drawn from multistate empirical valence bond simulations of the bulk liquid to calculate the infrared spectrum of the aqueous proton complex. Incorporation of anharmonic effects to the vibrational potential and quantum mechanical treatment of the proton produces a better agreement to the infrared spectrum compared to that of the double-harmonic approximation. We assess the correlation of the proton stretching mode with different atomistic coordinates, finding the best correlation with ⟨R_OH⟩, the expectation value of the proton-oxygen distance R_OH. We also decompose the IR spectrum based on normal mode vibrations and ⟨R_OH⟩ to provide insight on how different frequency regions in the continuum report on different configurations, vibrational modes, and mode couplings.

Anharmonic Vibrational Analysis of Biomolecules and Solvated Molecules Using Hybrid QM/MM Computations

Quantum mechanics/molecular mechanics (QM/MM) calculations are applied for anharmonic vibrational analyses of biomolecules and solvated molecules. The QM/MM method is implemented into a molecular dynamics (MD) program, GENESIS, by interfacing with external electronic structure programs. Following the geometry optimization and the harmonic normal-mode analysis based on a partial Hessian, the anharmonic potential energy surface (PES) is generated from QM/MM energies and gradients calculated at grid points. The PES is used for vibrational self-consistent field (VSCF) and post-VSCF calculations to compute the vibrational spectrum. The method is first applied to a phosphate ion in solution. With both the ion and neighboring water molecules taken as a QM region, IR spectra of representative hydration structures are calculated by the second-order vibrational quasi-degenerate perturbation theory (VQDPT2) at the level of B3LYP/cc-pVTZ and TIP3P force field. A weight-average of IR spectra over the structures reproduces the experimental spectrum with a mean absolute deviation of 16 cm^–1. Then, the method is applied to an enzyme, P450 nitric oxide reductase (P450nor), with the NO molecule bound to a ferric (Fe^III) heme. Starting from snapshot structures obtained from MD simulations of P450nor in solution, QM/MM calculations have been carried out at the level of B3LYP-D3/def2-SVP(D). The spin state of Fe^III(NO) is likely a closed-shell singlet state based on a ratio of N–O and Fe–NO stretching frequencies (ν_N–O and ν_Fe–NO) calculated for closed- and open-shell singlet states. The calculated ν_N–O and ν_Fe–NO overestimate the experimental ones by 120 and 75 cm^–1, respectively. The electronic structure and solvation of Fe^III(NO) affect the structure around the heme of P450nor leading to an increase in ν_N–O and ν_Fe–NO.

Quantum approaches to vibrational dynamics and spectroscopy: is ease of interpretation sacrificed as rigor increases?

The subject of this Perspective is quantum approaches, beyond the harmonic approximation, to vibrational dynamics and IR spectroscopy.

Molecular Origin of the Vibrational Structure of Ice Ih

An unambiguous assignment of the vibrational spectra of ice I_h remains a matter of debate. This study demonstrates that an accurate representation of many-body interactions between water molecules, combined with an explicit treatment of nuclear quantum effects through many-body molecular dynamics (MB-MD), leads to a unified interpretation of the vibrational spectra of ice I_h in terms of the structure and dynamics of the underlying hydrogen-bond network. All features of the infrared and Raman spectra in the OH stretching region can be unambiguously assigned by taking into account both the symmetry and the delocalized nature of the lattice vibrations as well as the local electrostatic environment experienced by each water molecule within the crystal. The high level of agreement with experiment raises prospects for predictive MB-MD simulations that, complementing analogous measurements, will provide molecular-level insights into fundamental processes taking place in bulk ice and on ice surfaces under different thermodynamic conditions.

The Melting Temperature of Liquid Water with the Effective Fragment Potential

The direct simulation of the solid-liquid water interface with the effective fragment potential (EFP) via the constant enthalpy and pressure (NPH) ensemble was used to estimate the melting temperature (T(m)) of ice-I(h). Initial configurations and velocities, taken from equilibrated constant pressure and temperature (NPT) simulations at P = 1 atm and T = 305 K, 325 K and 399 K, respectively, yielded corresponding T(m) values of 378 ± 16 K, 382 ± 14 K and 384 ± 15 K. These estimates are consistently higher than experiment, albeit to the same degree as previously reported estimates using density functional theory (DFT)-based Born-Oppenheimer simulations with the Becke-Lee-Yang-Parr functional plus dispersion corrections (BLYP-D).

Quantum calculations of the IR spectrum of liquid water using ab initio and model potential and dipole moment surfaces and comparison with experiment

The calculation and characterization of the IR spectrum of liquid water have remained a challenge for theory. In this paper, we address this challenge using a combination of ab initio approaches, namely, a quantum treatment of IR spectrum using the ab initio WHBB water potential energy surface and a refined ab initio dipole moment surface. The quantum treatment is based on the embedded local monomer method, in which the three intramolecular modes of each embedded H2O monomer are fully coupled and also coupled singly to each of six intermolecular modes. The new dipole moment surface consists of a previous spectroscopically accurate 1-body dipole moment surface and a newly fitted ab initio intrinsic 2-body dipole moment. A detailed analysis of the new dipole moment surface in terms of the coordinate dependence of the effective atomic charges is done along with tests of it for the water dimer and prism hexamer double-harmonic spectra against direct ab initio calculations. The liquid configurations are taken from previous molecular dynamics calculations of Skinner and co-workers, using the TIP4P plus E3B rigid monomer water potential. The IR spectrum of water at 300 K in the range of 0-4000 cm(-1) is calculated and compared with experiment, using the ab initio WHBB potential and new ab initio dipole moment, the q-TIP4P/F potential, which has a fixed-charged description of the dipole moment, and the TTM3-F potential and dipole moment surfaces. The newly calculated ab initio spectrum is in very good agreement with experiment throughout the above spectral range, both in band positions and intensities. This contrasts to results with the other potentials and dipole moments, especially the fixed-charge q-TIP4P/F model, which gives unrealistic intensities. The calculated ab initio spectrum is analyzed by examining the contribution of various transitions to each band.

Ab Initio Quantum Approaches to the IR Spectroscopy of Water and Hydrates

This Perspective highlights progress in ab initio quantum approaches to IR spectroscopy of water and hydrates. Here, "ab initio" refers to many-body potentials and dipole moment surfaces for flexible water and hydrates. Specifically, these are mathematical representations of two-body and three-body interactions based on permutationally invariant fitting of tens of thousands of ab initio electronic energies, a spectroscopically accurate one-body monomer potential, and four- and higher-body interactions described by the long-range interactions incorporated into, for example, the TTM3-F family of potentials. There are currently two such potentials of this type, denoted WHBB and MB-pol, which are being used in expanding applications. Here, the focus is on infrared spectroscopy, using the WHBB potential and dipole moment surface, with an embedded, local monomer quantum method to obtain vibrational energies and dipole transition moments. Comparisons are also made with the popular q-TIP4P/F potential. Brief mention is made of an application to small HCl-H2O clusters.

“Plug and play” full-dimensional ab initio potential energy and dipole moment surfaces and anharmonic vibrational analysis for CH4–H2O

The first full-dimensional potential energy surface of CH₄–H₂O dimer is presented, and vibrational analysis of this dimer is performed.

Local-monomer calculations of the intramolecular IR spectra of the cage and prism isomers of HOD(D2O)5 and HOD and D2O ice Ih

Dilute mixtures of HOD in pure H2O and D2O ices and liquid have been used by experimentalists to focus on the spectrum and vibrational dynamics of the local OH and OD stretches and bend of HOD in these complex and highly heterogeneous environments. The hexamer version of the mixture is HOD(D2O)5. The cage isomer of this cluster was recently studied and analyzed theoretically using local-mode calculations of the IR spectrum by Skinner and co-workers. This and the further possibility of experimental investigation of this cluster have stimulated us to study HOD(D2O)5 using the three-mode, local-monomer model, with the ab initio WHBB dipole moment and potential energy surfaces. Both the cage and prism isomers of this cluster are considered. In addition to providing additional insight into the HOD portion of the spectrum, the spectral signatures of each D2O are also presented in the range of 1000-4000 cm(-1). The OH stretch bands of both the prism and cage isotopomers exhibit rich structures in the range of 3100-3700 cm(-1) that are indicative of the position of the HOD in these hexamers. A preliminary investigation of the site preference of the HOD is also reported for both cage and prism HOD(D2O)5 using an harmonic zero-point energy analysis of the entire cluster. This indicates that the energies of free-OH sites are lower than the ones of H-bonded OH sites. Finally, following our earlier work on the IR spectra of H2O ice models, we present IR spectra of pure D2O and HOD.

Effects of Zero-Point Delocalization on the Vibrational Frequencies of Mixed HCl and Water Clusters

We demonstrate the significant effect that large-amplitude zero-point vibrational motion can have on the high-frequency fundamental vibrations of molecular clusters, specifically small (HCl)n-(H2O)m clusters. Calculations were conducted on a many-body potential, constructed from a mix of new and previously reported semiempirical and high-level ab initio potentials. Diffusion Monte Carlo simulations were performed to determine ground-state wave functions. Visualization of these wave functions indicates that the clusters exhibit delocalized ground states spanning multiple stationary point geometries. The ground states are best characterized by planar ring configurations, despite the clusters taking nonplanar configurations at their global minima. Vibrational calculations were performed at the global minima and the Diffusion Monte Carlo predicted configurations and also using an approach that spans multiple stationary points along a rectilinear normal-mode reaction path. Significantly better agreement was observed between the calculated vibrational frequencies and experimental peak positions when the delocalized ground state was accounted for.

Vibrational analysis of an ice Ih model from 0 to 4000 cm(-1) using the ab initio WHBB potential energy surface

We present an analysis of the vibrational modes of a model of hexagonal ice, ice Ih, comprised of 192 monomers with a core region of 105 monomers, using the ab initio WHBB potential energy surface [Wang, Y.; Shepler, B.; Braams, B.; Bowman, J. M. J. Chem. Phys. 2011, 134, 094509]. A standard normal-mode analysis and a local-monomer normal-mode analysis of 105 core monomers are performed to obtain harmonic frequencies and state densities of the "pseudo-translation" (0-400 cm(-1)), "libration" (500-1100 cm(-1)), monomer bend fundamental (~1600 cm), and O-H stretch (~3000-3700 cm(-1)) bands. In addition, the coupled local-monomer model is used to obtain the vibrational density of states in the bend fundamental and O-H stretch regions. The harmonic and local-monomer vibrational density of states obtained from core monomers are in good agreement with those of inelastic neutron scattering spectra, especially the latter, which accounts for anharmonic coupling of monomer modes. Full deuteration is also considered, and the vibrational density of states is again compared to experiment, where good agreement is found.

IR Spectra of the Water Hexamer: Theory, with Inclusion of the Monomer Bend Overtone, and Experiment Are in Agreement

Signature IR spectra of isomers of the water hexamer in the spectral range 3000-3800 cm(-1) have been reported by experimentalists, but crucial theoretical interpretation has still not been definitive. Using ab initio potential and dipole moment surfaces and a fully coupled quantum treatment of the intramolecular modes, the ring and book are assigned to spectra obtained in the He nanodroplet and Ar tagging experiments, respectively. The overtone of the intramolecular bend at ca. 3200 cm(-1) is a new calculated feature that completes an important missing piece in previous experimental and theoretical comparisons and leads to a consistent assignment of these two experimental spectra. Calculated IR spectra for the lowest energy forms of the water heptamer and octomer are also presented and compared to experiment. In all the calculated spectra, the bend overtone is demonstrated to be a noticeable feature, and this is one important conclusion from the work. Also, the danger in using scaled double-harmonic spectra to assign spectra is demonstrated.