arXiv subjects

Oliver Kühn

Publications and source records attributed to Oliver Kühn.

64 records · Page 4Linked to original sources

arXiv2011

Path Integral Approach to the Calculation of Reaction Rates for a Reaction Coordinate Coupled to a Dual Harmonic Bath

We present a new method for the numerical calculation of canonical reaction rate constants in complex molecular systems, which is based on a path integral formulation of the flux-flux correlation function. Central is the partitioning of the total system into a relevant part coupled to a dual bath. The latter consists of two parts: First, a set of strongly coupled harmonic modes, describing, for example, intramolecular degrees of freedom. They are treated on the basis of a reaction surface Hamiltonian approach. Second, a set of bath modes mimicking an unspecific environment modeled by means of a continuous spectral density. After deriving a set of general equations expressing the canonical rate constant in terms of appropriate influence functionals, several approximations are introduced to provide an efficient numerical implementation. Results for an initial application to the H-transfer in 6-Aminofulvene-1-aldimine are discussed.

physics.chem-ph↗

arXiv2011

Carbonyl Vibrational Wave Packet Circulation in Mn$_2$(CO)$_{10}$ Driven by Ultrashort Polarized Laser Pulses

The excitation of the degenerate $E_1$ carbonyl stretching vibrations in dimanganese decacarbonyl is shown to trigger wave packet circulation in the subspace of these two modes. On the time scale of about 5 picoseconds intramolecular anharmonic couplings do not cause appreciable disturbance, even under conditions where the two $E_1$ modes are excited by up to about two vibrational quanta each. The compactness of the circulating wave packet is shown to depend strongly on the excitation conditions such as pulse duration and field strength. Numerical results for the solution of the seven-dimensional vibrational Schrödinger equation are obtained for a density functional theory based potential energy surface and using the multi-configuration time-dependent Hartree method.

quant-ph↗

arXiv2011

Long-range Energy Transfer and Ionization in Extended Quantum Systems Driven by Ultrashort Spatially Shaped Laser Pulses

The processes of ionization and energy transfer in a quantum system composed of two distant H atoms with an initial internuclear separation of 100 atomic units (5.29 nm) have been studied by the numerical solution of the time-dependent Schrödinger equation beyond the Born-Oppenheimer approximation. Thereby it has been assumed that only one of the two H atoms was excited by temporally and spatially shaped laser pulses at various laser carrier frequencies. The quantum dynamics of the extended H-H system, which was taken to be initially either in an unentangled or an entangled ground state, has been explored within a linear three-dimensional model, including two z coordinates of the electrons and the internuclear distance R. An efficient energy transfer from the laser-excited H atom (atom A) to the other H atom (atom B) and the ionization of the latter have been found. It has been shown that the physical mechanisms of the energy transfer as well as of the ionization of atom B are the Coulomb attraction of the laser driven electron of atom A by the proton of atom B and a short-range Coulomb repulsion of the two electrons when their wave functions strongly overlap in the domain of atom B.

quant-ph↗

arXiv2011

Energy transfer among distant quantum systems in spatially shaped laser fields: Two H atoms with the internuclear separation of 5.29 nm (100 a.u.)

The quantum dynamics of two distant H atoms excited by ultrashort and spatially shaped laser pulses is studied by the numerical solution of the non-Born-Oppenheimer time-dependent Schrödinger equation within a three-dimensional (3D) model, including the internuclear distance R and the two z coordinates of the electrons, z1 and z2. The two 1D hydrogen atoms, A and B, are assumed to be initially in their ground states with a large (but otherwise arbitrary) internuclear separation of R = 100 a.u. (5.29 nm). Two types of a spatial envelope of a laser field linearly po- larized along the z-axis are considered: (i) a broad Gaussian envelope, such that atom A is excited by the laser field predominantly, and (ii) a narrow envelope, such that practically only atom A is excited by the laser field. With the laser carrier frequency ω = 1.0 a.u. and the pulse duration tp = 5 fs, in both cases an efficient energy transfer from atom A to atom B has been found. The ionization of atom B achieved mostly after the end of the laser pulse is close to or even higher than that of atom A. It is shown that with a narrow spatial envelope of the laser field, the underlying mechanisms of the energy transfer from A to B and the ionization of B are the Coulomb attraction of the laser driven electron by the proton of atom B and a short-range Coulomb repulsion of the two electrons when their wave functions significantly overlap in the domain of atom B. In the case of a broad Gaussian spatial envelope of the laser field, the opposite process also occurs, but with smaller probability: the energy is transferred from the weakly excited atom B to atom A, and the ionization of atom A is also induced by the electron-electron repulsion in the domain of atom A due to a strong overlap of the electronic wave functions.

quant-ph↗

arXiv2009

Infrared Laser Driven Double Proton Transfer. An Optimal Control Theory Study

Laser control of ultrafast double proton transfer is investigated for a two-dimensional model system describing stepwise and concerted transfer pathways. The pulse design has been done by employing optimal control theory in combination with the multiconfiguration time-dependent Hartree wave packet propagation. The obtained laser fields correspond to multiple pump-dump pulse sequences. Special emphasis is paid to the relative importance of stepwise and concerted transfer pathways for the driven wave packet and its dependence on the parameters of the model Hamiltonian as well as on the propagation time. While stepwise transfer is dominating in all cases considered, for high barrier systems concerted transfer proceeding via tunneling can make a contribution.

physics.chem-ph↗

arXiv2009

IR Spectrum of the O-H$...$O Hydrogen Bond of Phthalic Acid Monomethylester in Gas Phase and in CCl$_4$ Solution

The absorption spectrum of the title compound in the spectral range of the Hydrogen-bonded OH-stretching vibration has been investigated using a five-dimensional gas phase model as well as a QM/MM classical molecular dynamics simulation in solution. The gas phase model predicts a Fermi-resonance between the OH-stretching fundamental and the first OH-bending overtone transition with considerable oscillator strength redistribution. The anharmonic coupling to a low-frequency vibration of the Hydrogen bond leading to a vibrational progression is studied within a diabatic potential energy curve model. The condensed phase simulation of the dipole-dipole correlation function results in a broad band in the 3000 \cm region in good agreement with experimental data. Further, weaker absorption features around 2600 \cm have been identified as being due to motion of the Hydrogen within the Hydrogen bond.

physics.chem-ph↗

arXiv2009

QM/MM Simulation of the Hydrogen Bond Dynamics of an Adenine:Uracil Base Pair in Solution. Geometric Correlations and Infrared Spectrum

Hybrid QM(DFT)/MM molecular dynamics simulations have been carried out for the Watson-Crick base pair of 9-ethyl-8-phenyladenine and 1-cyclohexyluracil in deuterochloroform solution at room temperature. Trajectories are analyzed putting special attention to the geometric correlations of the $\NHN$ and $\NHO$ hydrogen bonds in the base pair. Further, based on empirical correlations between the hydrogen bond bond length and the fundamental NH stretching frequency its fluctuations are obtained along the trajectory. Using the time dependent frequencies the infrared lineshape is determined assuming the validity of a second order cumulant expansion. The deviations for the fundamental transition frequencies are calculated to amount to less than 2% as compared with experiment. The width of the spectrum for the $\NHN$ bond is in reasonable agreement with experiment while that for the $\NHO$ case is underestimated by the present model. Comparing the performance of different pseudopotentials it is found that the Troullier-Martins pseudopotential with a 70 Ry cut-off yields the best agreement.

physics.chem-ph↗

arXiv2008

A Concise Method for Kinetic Energy Quantisation

We present a straightforward method for obtaining exact classical and quantum molecular Hamiltonians in terms of arbitrary coordinates. As compared to other approaches the resulting expression are rather compact, the physical meaning of each quantity is quite transparent, and in some cases the calculation effort will be greatly reduced. We also investigate systems with constrains to find the suggested method to be applicable in contrast to most conventional approaches to kinetic energy operators which cannot directly be applied to constrained systems. Two examples are discussed in detail, the monohydrated hydroxide anion and the protonated ammonia dimer.

physics.chem-ph↗

arXiv2008

QM/MM Lineshape Simulation of the Hydrogen-bonded Uracil NH Stretching Vibration of the Adenine:Uracil Base Pair in CDCl$_3$

A hybrid Car-Parrinello QM/MM molecular dynamics simulation has been carried out for the Watson-Crick base pair of 9-ethyl-8-phenyladenine and 1-cyclohexyluracil in deuterochloroform solution at room temperature. The resulting trajectory is analyzed putting emphasis on the N-H$...$N Hydrogen bond geometry. Using an empirical correlation between the $\NN$-distance and the fundamental NH-stretching frequency, the time-dependence of this energy gap along the trajectory is obtained. From the gap-correlation function we determine the infrared absorption spectrum using lineshape theory in combination with a multimode oscillator model. The obtained average transition frequency and the width of the spectrum is in reasonable agreement with recent experimental data.

physics.chem-ph↗

arXiv2008

A full-dimensional quantum dynamical study of the vibrational ground state of $H_3O_2^-$ and its isotopomers

We investigated the effect of deuteration on the vibrational ground state of the hydrated hydroxide anion using a nine-dimensional quantum dynamical model for the case of J=0. The propagation of the nuclear wave function has been performed with the multi-configuration time-dependent Hartree method which yielded zero-point energies for the normal and fully deuterated species in quantitative agreement with previous diffusion Monte Carlo calculations. According to the zero-point energy the isotopomers having the hydrogen atom in the bridging position are more stable by about 1 kJ/mol as compared to the deuterium case. This holds irrespective of the deuteration state of the two OH groups. We also report the secondary geometric H/D isotope effect on the O--O distance which amounts to an elongation of about 0.005 A for the symmetric isotopomers and 0.009 A in the asymmetric case. Finally, we explore the isotopomer sensitivity of the ground state tunneling splitting due to the torsional motion of the two OH groups.

physics.chem-ph↗