  Viscosity driven bar mode secular instabilities of rapidly rotating neutron
stars are studied using LORENE/Nrotstar code. These instabilities set a more
rigorous limit to the rotation frequency of neutron star than the Kepler
frequency/mass shedding limit. The procedure employed in the code comprises of
perturbing an axisymmetric and stationary configuration of a neutron star and
studying its evolution by constructing a series of triaxial quasi-equilibrium
configurations. Symmetry breaking point was found out for Polytropic as well as
10 realistic Equations of states (EOS) from the CompOSE database. The concept
of piecewise polytropic EOSs has been used to comprehend the rotational
instability of Realistic EOSs and validated with 19 different Realistic EOSs
from CompOSE. The possibility of detecting quasi-periodic gravitational waves
from viscosity driven instability with ground based LIGO/VIRGO interferometers
is also discussed very briefly.
