On the Herman-Kluk Semiclassical Approximation
Résumé
For a subquadratic symbol $H$ on $\R^d\times\R^d = T^*(\R^d)$, the quantum propagator of the time dependent Schrödinger equation $i\hbar\frac{\partial\psi}{\partial t} = \hat H\psi$ is a Semiclassical Fourier-Integral Operator when $\hat H=H(x,\hbar D_x)$ ($\hbar$-Weyl quantization of $H$). Its Schwartz kernel is describe by a quadratic phase and an amplitude. At every time $t$, when $\hbar$ is small, it is ``essentially supported" in a neighborhood of the graph of the classical flow generated by $H$, with a full uniform asymptotic expansion in $\hbar$ for the amplitude.\\ In this paper our goal is to revisit this well known and fondamental result with emphasis on the flexibility for the choice of a quadratic complex phase function and on global $L^2$ estimates when $\hbar$ is small and time $t$ is large. One of the simplest choice of the phase is known in chemical physics as Herman-Kluk formula. Moreover we prove that the semiclassical expansion for the propagator is valid for $\vert t\vert << \frac{1}{4\delta}\vert\log\hbar\vert$ where $\delta>0$ is a stability parameter for the classical system.
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