Gaussian-based multiconfiguration time-dependent Hartree: A two-layer approach. II. Application to vibrational energy transport in a molecular chain.

We report on first applications of the Two-Layer Gaussian-based Multi-Configuration Time-Dependent Hartree (2L-GMCTDH) method [Römer et al., J. Chem. Phys. 138, 064106 (2013)] for high-dimensional quantum propagation using variational Gaussian basis sets. This method circumvents the limitations of conventional variational Gaussian wavepacket (GWP) methods by introducing a hierarchical wavefunction representation with a fully flexible first layer composed of orthogonal single-particle functions, which are in turn expressed as superpositions of GWPs of fixed width. The method is applied to a model Hamiltonian describing vibrational energy transport through a molecular chain. The model combines bilinear site-to-site couplings with site-local couplings induced by cubic anharmonicities. We report on simulation results for realizations comprising 5 sites with 35 vibrational modes and 18 sites with 90 vibrational modes, which are shown to be in excellent agreement with reference calculations by the Multi-Layer MCTDH method.

Gaussian-based multiconfiguration time-dependent Hartree: A two-layer approach. III. Application to nonadiabatic dynamics in a charge transfer complex.

In this paper, we report on first applications of the Two-Layer Gaussian-based Multi-Configuration Time-Dependent Hartree (2L-GMCTDH) method to nonadiabatic dynamics. Simulations of ultrafast, coherent charge transfer dynamics are performed for a two-state linear vibronic coupling model describing an oligothiophene-fullerene charge transfer complex, for system dimensions ranging from 20 to 100 modes. Different variants of the state-dependent 2L-GMCTDH propagation are assessed, notably single-set and multi-set versions, along with a third hybrid variant. It is shown that the method is suitable to perform accurate and efficient nonadiabatic dynamics simulations in many dimensions.

Gaussian-based multiconfiguration time-dependent Hartree: a two-layer approach. I. Theory.

We describe a novel two-layer variant of the Gaussian-based multiconfiguration time-dependent Hartree (G-MCTDH) approach which improves on the performance and convergence properties of quantum propagation based on variationally evolving frozen Gaussians (FGs). While the standard scheme uses factorizable multi-dimensional FGs, the present approach combines these into flexible, MCTDH-like single-particle functions. At the same time, the expensive variational evolution of the Gaussian parameters is reduced to low-dimensional subspaces. As a result, the novel scheme significantly alleviates the current bottleneck to accurate propagation in G-MCTDH and its variational multiconfigurational Gaussian (vMCG) variant. Since the first-layer single-particle functions are chosen to be orthogonal, the present approach can be straightforwardly combined with existing multi-layer MCTDH schemes.

The photodynamics of ethylene: a surface-hopping study on structural aspects.

Simulations of the photodynamics of ethylene were carried out by employing the semiempirical direct trajectory with surface hopping method in order to assess quantitatively the importance of different regions of the S(2)S(1) and S(1)S(0) crossing seams. The results show that during the first 50 fs after a vertical photoexcitation to the pipi(*) state, the nonadiabatic coupling between the S(1) and the S(2) states produces a recurrence pattern of oscillation of the populations in these states. Within the first 100 fs, the S(1) state population spans a limited region of the configuration space between the initial geometries and the twisted-pyramidalized minimum on the crossing seam (MXS). Depending on the way of counting, about 50% of the S(1)-->S(0) transitions occur in the pyramidalized region of the crossing seam, but not necessarily close to the MXS. The remaining 50% occurs in the H-migration and ethylidene regions. Our analysis shows that the ethylidene region becomes more important in later stages of the dynamics when the flux of trajectories that was not effectively converted to the ground state in the pyramidalized region starts to reach this part of the configuration space. The excited-state nonadiabatic dynamics could be employed to generate suitable initial phase space distributions for the hot-ethylene ground-state kinetic studies.