arXiv subjects

Kasra Asnaashari

Publications and source records attributed to Kasra Asnaashari.

4 recordsLinked to original sources

arXiv2026

A partially linearized mapping approach to surface hopping (MASH-PLDM) for nonadiabatic dynamics and nonlinear spectroscopy

We derive a partially linearized version of the mapping approach to surface hopping (MASH) that provides a new method for simulating nonadiabatic dynamics in molecular systems. As in the original MASH formalism, trajectories travel on adiabatic surfaces with hops determined by the dynamics of a spin vector on the Bloch sphere. However, because the new approach is based on the partially linearized density matrix (PLDM), it has two spin vectors, which may be in the same or different hemispheres of the Bloch sphere. The former scenario describes an electronic population moving on a single adiabatic surface, whereas the latter describes a coherence moving on the average surface. We show that this new method, called MASH-PLDM, leads to improved results over the original MASH dynamics for a variety of scattering problems and spin-boson models, especially in cases where coherences play an important role. A further benefit of the new methodology is that it can treat multi-time correlation functions such as those required for nonlinear spectroscopy. We demonstrate this capability with a proof-of-principle simulation of time-resolved pump-probe spectroscopy.

physics.chem-ph↗

arXiv2026

Open quantum-classical systems: A hybrid MASH master equation

We propose a method which combines the quantum-classical mapping approach to surface hopping (MASH) with the dissipative quantum dynamics of the Lindblad master equation. Like conventional surface-hopping methods, our approach is based on classical trajectories coupled to the dynamics of a quantum subsystem. However, instead of evolving the subsystem wavefunction according to the time-dependent Schrödinger equation, we use stochastic quantum trajectories derived from secular Redfield theory. This enables the simulation of open quantum systems coupled simultaneously to Markovian quantum baths and anharmonic non-Markovian classical degrees of freedom. Applications to the spin--boson model and to the cavity-enhanced fluorescence of an electronically nonadiabatic molecule show excellent agreement with fully quantum-mechanical benchmarks.

quant-ph↗

arXiv2024

Time-reversible implementation of MASH for efficient nonadiabatic molecular dynamics

In this work, we describe various improved implementations of the mapping approach to surface hopping (MASH) for simulating nonadiabatic dynamics. These include time-reversible and piecewise-continuous integrators, which is only formally possible because of the deterministic nature of the underlying MASH equations of motion. The new algorithms allow for the use of either wave-function overlaps or nonadiabatic coupling vectors to propagate the spin, which encodes the electronic state. For a given time-step, $Δt$, it is demonstrated that the global error for these methods is $\mathcal{O}(Δt^2)$ compared to the $\mathcal{O}(Δt)$ error of standard implementations. This allows larger time-steps to be used for a desired error tolerance, or conversely, more accurate observables given a fixed value of $Δt$. The newly developed integrators thus provide further advantages for the MASH method, demonstrating that it can be implemented more efficiently than other surface-hopping approaches, which cannot construct time-reversible integrators due to their stochastic nature.

physics.chem-ph↗

arXiv2024

A MASH simulation of the photoexcited dynamics of cyclobutanone

In response to a community prediction challenge, we simulate the nonadiabatic dynamics of cyclobutanone using the mapping approach to surface hopping (MASH). We consider the first 500 fs of relaxation following photo-excitation to the S2 state and predict the corresponding time-resolved electron-diffraction signal that will be measured by the planned experiment. 397 ab-initio trajectories were obtained on the fly with state-averaged complete active space self-consistent field (SA-CASSCF) using a (12,11) active space. To obtain an estimate of the potential systematic error 198 of the trajectories were calculated using an aug-cc-pVDZ basis set and 199 with a 6-31+G* basis set. MASH is a recently proposed independent trajectory method for simulating nonadiabatic dynamics, originally derived for two-state problems. As there are three relevant electronic states in this system, we used a newly developed multi-state generalisation of MASH for the simulation: the uncoupled spheres multi-state MASH method (unSMASH). This study therefore serves both as an investigation of the photo-dissociation dynamics of cyclobutanone, and also as a demonstration of the applicability of unSMASH to ab-initio simulations. In line with previous experimental studies, we observe that the simulated dynamics is dominated by three sets of dissociation products, C3H6+CO, C2H4+C2H2O and C2H4+CH2+CO, and we interpret our predicted electron-diffraction signal in terms of the key features of the associated dissociation pathways.

physics.chem-ph↗