arXiv subjects

N. T. Maitra

Publications and source records attributed to N. T. Maitra.

5 recordsLinked to original sources

arXiv2016

Electronic Non-adiabatic Dynamics in Enhanced Ionization of Isotopologues of H$_2^+$ from the Exact Factorization Perspective

It was recently shown that the exact potential driving the electron's dynamics in enhanced ionization of H$_2^+$ can have large contributions arising from dynamical electron-nuclear correlation, going beyond what any electrostatics-based model can provide[1]. This potential is defined via the exact factorization of the molecular wavefunction that allows the construction of a Schrödinger equation for the electronic system, in which the potential contains exactly the effect of coupling to the nuclear system and any external fields. Here we study enhanced ionization in isotopologues of H$_2^+$ in order to investigate nuclear-mass-dependence of these terms for this process. We decompose the exact potential into components that naturally arise from the conditional wavefunction, and also into components arising from the marginal electronic wavefunction, and compare the performance of propagation on these different components as well as approximate potentials based on the quasi-static or Hartree approximation with the exact propagation. A quasiclassical analysis is presented to help analyse the structure of different non-electrostatic components to the potential driving the ionizing electron.

physics.atm-clus↗

arXiv2014

Charge-Transfer in Time-Dependent Density Functional Theory: Insights from the Asymmetric Hubbard Dimer

We show that an asymmetric two-fermion two-site Hubbard model illustrates the essential features of long-range charge-transfer dynamics in a real-space molecule. We apply a resonant field that transfers one fermion from one site to the other. Via constrained search we find the exact ground-state exchange-correlation functional, and use it to propagate the Kohn-Sham system, giving the first "adiabatically-exact" calculation of time-resolved charge-transfer. This propagation fails to properly transfer charge. We analyze why by comparing the exact and adiabatically-exact potentials and discuss the role of the derivative discontinuity. The implication for real-space molecules is that even the best possible adiabatic approximation, despite capturing non-local step features relevant to dissociation and charge-transfer excitations, cannot capture fully time-resolved charge-transfer dynamics.

physics.chem-ph↗

arXiv2013

Absence of Dynamical Steps in the Exact Correlation Potential in Linear Response

Recent work showed that the exact exchange-correlation potential of time-dependent density functional theory generically displays dynamical step structures. These have a spatially non-local and time-non-local dependence on the density in real time dynamics. The steps are missing in the usual approximations which consequently yield inaccurate dynamics. Yet these same approximations typically yield good linear response spectra. Here we investigate whether the steps appear in the linear response regime, when the response is calculated from a real-time dynamics simulation, by examining the exact correlation potential of model two-electron systems at various times. We find there are no step structures in regions where the system response is linear. Step structures appear in the correlation potential only in regions of space where the density response is non-linear; these regions, having exponentially small density, do not contribute to the observables measured in linear response.

physics.chem-ph↗

arXiv2010

Semiclassical Electron Correlation in Density-Matrix Time-Propagation

Lack of memory (locality in time) is a major limitation of almost all present time-dependent density functional approximations. By using semiclassical dynamics to compute correlation effects within a density-matrix functional approach, we incorporate memory, including initial-state dependence, as well as changing occupation numbers, and predict more observables in strong-field applications.

physics.chem-ph↗

arXiv1998

Quantizing Constrained Systems: New Perspectives

We consider quantum mechanics on constrained surfaces which have non-Euclidean metrics and variable Gaussian curvature. The old controversy about the ambiguities involving terms in the Hamiltonian of order hbar^2 multiplying the Gaussian curvature is addressed. We set out to clarify the matter by considering constraints to be the limits of large restoring forces as the constraint coordinates deviate from their constrained values. We find additional ambiguous terms of order hbar^2 involving freedom in the constraining potentials, demonstrating that the classical constrained Hamiltonian or Lagrangian cannot uniquely specify the quantization: the ambiguity of directly quantizing a constrained system is inherently unresolvable. However, there is never any problem with a physical quantum system, which cannot have infinite constraint forces and always fluctuates around the mean constraint values. The issue is addressed from the perspectives of adiabatic approximations in quantum mechanics, Feynman path integrals, and semiclassically in terms of adiabatic actions.

quant-ph↗