Applicability of the Caldeira–Leggett Model
to Vibrational Spectroscopy in Solution
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Abstract
Formulating a rigorous system–bath
partitioning approach
remains an open issue. In this context, the famous Caldeira–Leggett
model that enables quantum and classical treatment of Brownian motion
on equal footing has enjoyed popularity. Although this model is by
any means a useful theoretical tool, its ability to describe anharmonic
dynamics of real systems is often taken for granted. In this Letter,
we show that the mapping between a molecular system under study and
the model cannot be established in a self-consistent way, unless the
system part of the potential is taken effectively harmonic. Mathematically,
this implies that the mapping is not invertible. This “invertibility
problem” is not dependent on the peculiarities of particular
molecular systems and is rooted in the anharmonicity of the system
part of the Caldeira–Leggett model potential