The intermediate scattering function is interpreted as a correlation function of thermal wave packets of the scattering centers perturbed by the scattering particles at different times. A proof of concept is given at the example of ballistic moving centers. The ensuing numerical method is then illustrated at the example of CO adsorbed on Cu(100).

In this contribution, we investigate the non-Markovian relaxation dynamics of a vibrating system in contact with a structured environment. Numerical simulations of the vibrational relaxation dynamics of an adsorbate coupled to a bath of phonons are performed using the stochastic multiconfiguration time-dependent Hartree method. Non-Markovian effects arise from the partitioning of the system-bath interaction into explicit and implicit contributions. It is shown that only a small number of explicit bath modes is sufficient to capture the short-time non-Markovian dynamics, and that imposing a "Markovian closure" of the weakly coupled explicit bath allows other physical regimes for the vibrational relaxation dynamics with distinctive signatures to be assessed.

The numerical cost of variational methods suggests using perturbative approaches to determine the electronic structure of molecular systems. In this work, a sequential construction of effective Hamiltonians drives the definition of approximate model functions and energies in a multi-state Rayleigh-Schr & ouml;dinger perturbative scheme. A second step takes advantage of an updated partitioning of the Hamiltonian to perform a state-specific Brillouin-Wigner energy correction based on a well-tempered perturbation expansion. The multi-step RSBW method is exemplified on model-Hamiltonians to stress its robustness, efficiency and applicability to spectroscopy determination.

The mean square displacement ⟨( x(t)-x(0)) ^2⟩ _ of the position x of a free particle of mass m is evaluated quantum mechanically in terms of thermal Gaussian wave packets (Marquardt in Mol Phys 119(17-18): e1974110, https://doi.org/10.1080/00268976.2021.1974110 , 2021). Such states represent a particle at thermal equilibrium. The approach allows us to address the evaluation of the mean square displacement in the continuum as well as under periodic boundary conditions. In the latter case, an analytical expression is obtained which is identical to the expression obtained on the basis of plane waves (Marquardt in Mol Phys 19(17-18): e1971315, https://doi.org/10.1080/00268976.2021.1971315 , 2021). Results are discussed with respect to the quantum mechanical representation of observables related to the mean square displacement of particles.

The equivalence in one-electron quantum baths between the practical implementation of density matrix embedding theory (DMET) and the more recent Householder-transformed density matrix functional embedding theory has been shown previously in the standard but special case where the reference full-size (one-electron reduced) density matrix, from which the bath is constructed, is idempotent [S. Yalouz et al., J. Chem. Phys. 157, 214112 (2022)]. We prove mathematically that the equivalence remains valid when the density matrix is not idempotent anymore, thus allowing for the construction of correlated (one-electron) quantum baths. A density-matrix functional exactification of DMET is derived within the present unified quantum embedding formalism. Numerical examples reveal that the embedding cluster can be quite sensitive to the level of density-matrix functional approximation used for computing the reference density matrix.

In this article, we explore the dissipation dynamics of a strongly coupled multidimensional system in contact with a Markovian bath, following a system-bath approach. We use in this endeavor the recently developed stochastic multi-configuration time-dependent Hartree approach within the Monte Carlo wave packet formalism [S. Mandal et al., J. Chem. Phys. 156, 094109 (2022)]. The method proved to yield thermalized ensembles of wave packets when intramolecular coupling is weak. To treat strongly coupled systems, new Lindblad dissipative operators are constructed as linear combinations of the system coordinates and associated momenta. These are obtained by a unitary transformation to a normal mode representation, which reduces intermode coupling up to second order. Additionally, we use combinations of generalized raising/lowering operators to enforce the Boltzmann distribution in the dissipation operators, which yield perfect thermalization in the harmonic limit. The two ansatz are tested using a model two-dimensional Hamiltonian, parameterized to disentangle the effects of intramolecular potential coupling, of strong mode mixing observed in Fermi resonances, and of anharmonicity.

In this paper, multidimensional dissipative quantum dynamics is studied within a system-bath approach in the Markovian regime using a model Lindblad operator. We report on the implementation of a Monte Carlo wave packet algorithm in the Heidelberg version of the Multi-Configuration Time-Dependent Hartree (MCTDH) program package, which is henceforth extended to treat stochastic dissipative dynamics. The Lindblad operator is represented as a sum of products of one-dimensional operators. The new form of the operator is not restricted to the MCTDH formalism and could be used with other multidimensional quantum dynamical methods. As a benchmark system, a two-dimensional coupled oscillators model representing the internal stretch and the surface-molecule distance in the O2/Pt(111) system coupled to a Markovian bath of electron-hole-pairs is used. The simulations reveal the interplay between coherent intramolecular coupling due to anharmonic terms in the potential and incoherent relaxation due to coupling to an environment. It is found that thermalization of the system can be approximately achieved when the intramolecular coupling is weak.

We implemented an N-body potential for the Al-Co interactions and applied it to the o-Al13Co4 quasicrystalline approximant. We show its ability to model this complex compound in the presence of point and extended defects (atomic vacancies and surfaces). The importance of stress relaxation in vacancy formation is highlighted through the mapping of local pressures in the bulk compound. Thanks to the many body character of the potential, the surfaces could be investigated which was not done before in atomistic studies of this complex phase. Our classical simulations point up the competition between preserving the cohesion by minimizing the number of broken bonds and avoiding the presence of Co atoms at the surface. This study opens the way to large scale simulations of phenomena involving complex metallic alloys in particular at their surfaces.