  Valuable dynamical and structural information about neat liquid DMSO at
ambient conditions can be obtained through study of low frequency vibrations in
the far infrared (FIR), that is, terahertz regime. For DMSO, collective
excitations as well as single molecule stretches and bends have been measured
by different kinds of experiments such as OHD-RIKES and terahertz spectroscopy.
In the present work we investigate the intermolecular vibrational spectrum of
DMSO through three different computational techniques namely (i) the far-infra
red spectrum obtained through Fourier transform of total dipole moment auto
time correlation function, (ii) from Fourier transform of the translational and
angular velocity time autocorrelation functions and a (iii) quenched normal
mode analysis of the parent liquid at 300K. The three spectrum, although
exhibit differences among each other, reveal similar features which are in
good, semi-quantitative, agreement with experimental results. Study of
participation ratio of the density of states obtained from normal mode analysis
shows that the broad spectrum around 100 cm-1 involves collective oscillations
of 300-400 molecules. Dipolar solvation dynamics exhibit ultrafast energy
relaxation (dipolar solvation dynamics) with initial time correlation function
around 140 fs which can be attributed to the coupling to the collective
excitations. We compare properties of DMSO with those of water vis-a-vis the
existence of the low frequency collective modes. Lastly, we find that the
collective excitation spectrum exhibits strong temperature dependence.
