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arXiv:1603.03963

Quantum Dynamics in Phase space using the Biorthogonal von Neumann bases: Algorithmic Considerations

Shai Machnes, Elie Assémat, David J. Tannor

Published Mar 12, 2016• 2 citations

Abstract

The von Neumann lattice refers to a discrete basis of Gaussians located on a lattice in phase space. It provides an attractive approach for solving quantum mechanical problems, allowing the pruning of tensor-product basis sets using phase space considerations. In a series of recent articles Shimshovitz et al. [Phys. Rev. Lett. 109 7 (2012)], Takemoto et al. [Journal of Chemical Physics 137 1 (2012)] Machnes et al. [Journal of Chemical Physics, accepted (2016)]), we have introduced two key new elements into the method: a formalism for converging the basis and for efficient pruning by use of the biorthogonal basis. In this paper we review the key components of the theory and then present new, efficient and parallelizable iterative algorithms for solving the time-independent and time-dependent Schrödinger equations. The algorithms dynamically determine the active reduced basis iteratively without resorting to classical analogs. These algorithmic developments, combined with the previous formal developments, allow quantum dynamics to be performed directly and economically in phase space. We provide two illustrative examples: double-well tunneling and double ionization of helium.

DOI: 10.48550/arxiv.1603.03963

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The von Neumann lattice refers to a discrete basis of Gaussians located on a lattice in phase space. It provides an attractive approach for solving quantum mechanical problems, allowing the pruning of tensor-product basis sets using phase space considerations. In a series of recent articles Shimshovitz et al. [Phys. Rev. Lett. 109 7 (2012)], Takemoto et al. [Journal of Chemical Physics 137 1 (2012)] Machnes et al. [Journal of Chemical Physics, accepted (2016)]), we have introduced two key new elements into the method: a formalism for converging the basis and for efficient pruning by use of the biorthogonal basis. In this paper we review the key components of the theory and then present new, efficient and parallelizable iterative algorithms for solving the time-independent and time-dependent Schrödinger equations. The algorithms dynamically determine the active reduced basis iteratively without resorting to classical analogs. These algorithmic developments, combined with the previous formal developments, allow quantum dynamics to be performed directly and economically in phase space. We provide two illustrative examples: double-well tunneling and double ionization of helium.

We describe the mathematical underpinnings of the biorthogonal von Neumann method for quantum mechanical simulations (PvB). In particular, we present a detailed discussion of the important issue of nonorthogonal projection onto subspaces of biorthogonal bases, and how this differs from orthogonal projection. We present various representations of the Schrödinger equation in the reduced basis and discuss their relative merits. We conclude with illustrative examples and a discussion of the outlook and challenges ahead for the PvB representation.

The time-dependent and time-independent Schrödinger equations have been numerically solved for several quantum mechanical problems with known analytical solutions using the basis-spline collocation algorithm. This algorithm has been demonstrated to be efficient and versatile. The results from these calculations illustrate the necessary numerical considerations required for solving both time-independent and time-dependent Schrödinger equations and form a basis that could be used for solving these and similar problems.

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A nonerturbative approach to such problems has been developed, based on a Liouville-space expansion of the solution of Heisenberg equation of motion for the relevant operators as a series of time-independent basis elements with time-dependent coefficients to be determined. Since the method is applicable to nonlinear dynamics of both classical and quantum systems, these two cases are treated in parallel. The necessary nonlinear dependence on initial conditions is introduced by allowing Liouvellian eigenvalues to depend on constants of motion. The formalism is illustrated by application to the anharmonic classical oscillator and the quantum Jaynes-Cummings model.