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Journal of Physics A Mathematical and General

Numerical evaluation of coherent-state path integrals in quantum dynamics

Bernd Burghardt, Joachim Stolze

Published Jan 1, 1999• 4 citations
Publisher

Abstract

The numerical evaluation of coherent-state path integrals for quantum dynamical problems is discussed for one-dimensional examples. To propagate an initial state, we use the normal and antinormal ordered coherent-state path integrals combined with a split-operator technique dividing the Hamiltonian into harmonic and anharmonic parts. For numerical purposes integrations must be approximated by quadrature formulae. This leads to a matrix multiplication scheme which is systematically tested for the double-well and Morse potentials. The method is accurate for propagation times much longer than the natural time scale of the system, and it allows for short as well as long time steps without loss of stability.

DOI
10.1088/0305-4470/32/11/004

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The numerical evaluation of coherent-state path integrals for quantum dynamical problems is discussed for one-dimensional examples. To propagate an initial state, we use the normal and antinormal ordered coherent-state path integrals combined with a split-operator technique dividing the Hamiltonian into harmonic and anharmonic parts. For numerical purposes integrations must be approximated by quadrature formulae. This leads to a matrix multiplication scheme which is systematically tested for the double-well and Morse potentials. The method is accurate for propagation times much longer than the natural time scale of the system, and it allows for short as well as long time steps without loss of stability.
We study the application of the semiclassical coherent-state path-integral technique to the problem of scattering. A numerical method based on initial-value trajectories and their stability information is proposed to solve the root search for the complex trajectories that determine the semiclassical propagator. Very good agreement between the numerically exact quantum and semiclassical results for the correlation function can be achieved for scattering from one-dimensional barrier as well as well-type potentials by using just one single complex trajectory at each time.
Recent progress in numerical methods for evaluating the real-time path integral in dissipative harmonic environments is reviewed. Quasi-adiabatic propagators constructed numerically allow convergence of the path integral with large time increments. Integration of the harmonic bath leads to path integral expressions that incorporate the exact dynamics of the quantum particle along the adiabatic path, with an influence functional that describes nonadiabatic corrections. The resulting quasi-adiabatic propagator path integral is evaluated by efficient system-specific quadratures in most regimes of parameter space, although some cases are handled by grid Monte Carlo sampling. Exploiting the finite span of nonlocal influence functional interactions characteristic of broad condensed phase spectra leads to an iterative scheme for calculating the path integral over arbitrary time lengths. No uncontrolled approximations are introduced, and the resulting methodology converges to the exact quantum result with modest amounts of computational power. Applications to tunneling dynamics in the condensed phase are described.
Various numerical integration schemes to calculate the propagation of a state following the time-dependent Schrodinger equation in the one dimensional position representation are presented and compared to each other. Three potentials have been used: a harmonic, a double-well and a zero potential. Eigenstates and a coherent state have been chosen as initial states. Special attention has been given to the long-time stability of the algorithms. These are: kinetic referenced split operator (KRSO), kinetic referenced Cayley (KRC), distributed approximating functions (DAF), Chebysheff expansion (CH), residuum minimization (RES), second-order differencing (SOD), an eigenstate expansion (EE) and a corrected kinetic referenced split operator (CKRSO). In addition, a speedup of the KRC and KRSO methods is presented which is specially suited when very few grid points are used. Numerical results are compared to analytically calculated values. Mixed classical/quantum mechanical simulations require a representation of the quantum state on a limited number of grid points, classical integration time steps of about one femtosecond and compatibility with methods to solve the time-ordering problem. For the considered potentials which differ quite essentially from the potentials used for scattering problems in particle physics, the EE method has been found to be faster, more accurate and more stable than the other methods if only a few grid points are required. Otherwise, good results have been obtained with KRC, KRSO, CH, DAF and RES. SOD has been found to be too slow, and CKRSO is not stable enough for long simulation times.