  Recent experiments and theory demonstrate that the the ground state
properties and chemical reactivity of molecules can be modified inside an
optical cavity. The vibrational strong or ultrastrong coupling results in the
formation of vibrational polaritons which are usually observed through infrared
spectra (IR). Here, we provide a theoretical framework to conduct
multidimensional quantum simulations of the infrared spectra when the molecule
is interacting with cavity modes. Taking single water molecule as an example,
combing with accurate potential energy and dipole moment surfaces, our
implemented cavity vibrational self-consistent field/virtual state
configuration interaction (cVSCF/VCI) is shown to be able to provide
quantitative predictions of the IR spectra when the molecule is inside or
outside the cavity. The spectral signatures of resonance splittings and
blue/red shift of certain bands are found to be highly related with the
frequency and polarization direction of the cavity modes. Further analyses of
the simulated spectra shows that polaritonic strong vibrational coupling
greatly induce the coupling between molecule's vibrational modes, indicating
the intramolecular vibrational energy transfer may be significantly accelerated
by the cavity.
