Alexander Witt

On the applicability of centroid and ring polymer path integral molecular dynamics for vibrational spectroscopy

Centroid molecular dynamics (CMD) and ring polymer molecular dynamics (RPMD) are two conceptually distinct extensions of path integral molecular dynamics that are able to generate approximate quantum dynamics of complex molecular systems. Both methods can be used to compute quasiclassical time correlation functions which have direct application in molecular spectroscopy; in particular, to infrared spectroscopy via dipole autocorrelation functions. The performance of both methods for computing vibrational spectra of several simple but representative molecular model systems is investigated systematically as a function of temperature and isotopic substitution. In this context both CMD and RPMD feature intrinsic problems which are quantified and investigated in detail. Based on the obtained results guidelines for using CMD and RPMD to compute infrared spectra of molecular systems are provided.

Methanol synthesis on ZnO(0001¯). I. Hydrogen coverage, charge state of oxygen vacancies, and chemical reactivity

Oxygen vacancies on ZnO(0001) have been proposed to be the catalytically active sites for methanol synthesis on pure ZnO. The charge state and thus the chemical reactivity of such vacancies on this polar O-terminated basal plane of ZnO is expected to be intimately connected to the degree of its hydroxylation in view of its Tasker type(3) unstable character. Here, the interplay between hydrogen adsorption and the thermodynamic stability of O vacancies in various charge states, corresponding formally to F(++), F(+), F(0), F(-), and F(--) centers, is investigated using electronic structure calculations. Assuming thermodynamic equilibrium of the defective surface with a hydrogen containing gas phase the thermodynamically most stable O vacancy type is determined as a function of temperature and pressure. For the adsorption of H(2) molecules at O vacancy sites it is found that the homolytic process leads to energetically more favorable structures than heterolytic adsorption and hydride formation. By homolytic adsorption and desorption one can switch between F(++), F(0), and F(--) or between F(+) and F(-), a process which is believed to occur during methanol synthesis. However, the barrier for heterolytic dissociation of H(2) at O vacancies is significantly lower compared to homolytic cleavage. Furthermore, the barrier for transforming hydridic hydrogen, i.e., ZnH species, to protonic hydrogen, i.e., OH species together with a reduction of ZnO itself, is quite high. This implies that hydridic H(-) species created as a result of heterolytic dissociation might have a long enough lifetime at O vacancies that they will be available for methanol synthesis. ZnH and OH vibrational frequencies have been computed in order to assist future experimental assignments.

Tailoring the Cu(100) work function by substituted benzenethiolate self-assembled monolayers.

The structure and electronic interface properties of five differently substituted benzenethiol based self-assembled monolayers (SAMs) on Cu(100) have been studied by means of low energy electron diffraction, thermal desorption spectroscopy, X-ray absorption spectroscopy (NEXAFS), and UV photoelectron spectroscopy. Because highly ordered SAMs are formed of which lateral density had been precisely determined, effective molecular dipole moments were derived from the measured work function shifts. These values are compared with gas phase dipole moments computed by quantum chemical calculations for the individual thiol molecules considering the molecular orientation determined from NEXAFS data. Furthermore, this comparison yields clear evidence for a coverage dependent depolarization effect of the adsorbed molecules within the SAMs.

Theoretical spectroscopy using molecular dynamics: theory and application to CH5+ and its isotopologues

Infrared spectroscopy is a powerful technique to unravel the structure and dynamics of molecular systems of ever increasing complexity. For isolated molecules in the gas phase theoretical approaches that directly rely on solving the Schrödinger equation, either approximately or quasi-exactly, are well established. A distinctly different approach to compute infrared spectra can be based on advanced molecular dynamics, itself being based on classical Newtonian dynamics, in conjunction with concurrent first principles electronic structure calculations. At variance with traditional methods, which are formulated in terms of the Schrödinger representation of quantum mechanics, the molecular dynamics approach stems from Heisenbergs representation and thus relies on computing thermal expectation values of time-correlation functions. Crucial in addition to generating the spectra themselves is their decomposition in terms of modes, which can be assigned to correlated atomic motion. This ab initio molecular dynamics route to compute infrared spectra, and its recent extension to quasiclassical techniques relying on approximate path integral dynamics, is covered in the review part of this Perspective. The usefulness of this unconventional approach, which can be generalized beyond infrared spectroscopy, is demonstrated in detail by applying the full machinery in computing and assigning the infrared spectra of protonated methane and its isotopologues. This particular molecule is often considered to be the most prominent member of the class of floppy or fluxional molecules. CH5(+) has been a longstanding challenge for theoretical infrared spectroscopy because it undergoes intricate large-amplitude motion, which is also reviewed. Molecular dynamics based infrared spectroscopy is general and can be applied to diverse systems such as molecular complexes in the gas phase, chromophores in biomolecular environments, and solute-solvent systems in the liquid phase.

Microsolvation of protonated methane: structures and energetics of CH5(+)(H2)n.

Effects of microsolvating CH(5)(+) with up to four H(2) molecules have been investigated in terms of structures and energies. For the smaller complexes, benchmark calculations have been carried out using MP2 and CCSD(T) with basis sets up to aug-cc-pV5Z quality and energies have been extrapolated to the infinite basis set limit. It is found that MP2 calculations using the aug-cc-pVQZ basis set or better yield robust reference data for both structures and energies. More than 30 stationary points including minima and first-order as well as second-order stationary points have been characterized by this method and are discussed in terms of solvation motifs. Finally, the performance of several density functionals has been assessed for this very demanding case. Popular GGA functionals such as BLYP and PBE fail, whereas the TPSS meta-GGA functional captures many structural and energetic aspects of microsolvation satisfactorily.

Ab Initio Path Integral Simulations of Floppy Molecular Systems

Protonated methane, \(\mbox{CH}_{5}^{+}\) , is one of the smallest representatives of the so–called floppy molecules whose treatment challenges both experiment and theory for decades. Recently, we succeeded in understanding the IR spectrum of the per–protonated parent system, i.e. isolated \(\mbox{CH}_{5}^{+}\) itself. More recently, the IR spectra of all its H/D isotopologues, i.e. \(\mbox{CD}_{5}^{+}\) , \(\mbox{CHD}_{4}^{+}\) , \(\mbox{CH}_{2}^{}\mbox{D}_{3}^{+}\) , \(\mbox{CH}_{3}^{}\mbox{D}_{2}^{+}\) , \(\mbox{CH}_{4}^{}\mbox{D}^{+}\) and \(\mbox{CH}_{5}^{+}\) have been measured in a tour de force experiment by our collaborators and now wait for interpretation. It has been shown both computationally and experimentally that nuclear quantum effects are crucial, which implies that they cannot be neglected when computing infrared spectra subject to H/D isotopic substitution. Thus, our investigations are carried out in the framework of ab initio path integral simulations together with the adiabatic centroid molecular dynamics extension which readily allow for nuclear quantum effects and yield access to the quasi–classical dynamics.