I was wondering how to perform this
$C(s) = \frac{1}{s} + \frac{(s + \zeta\omega_n) + \frac{\zeta}{\sqrt{1 + \zeta^2}}\omega_n\sqrt{1 - \zeta^2}}{(s + \zeta\omega_n)^2 + \omega^2_n(1 - \zeta^2)}$
to
$C(t) = 1 - e^{-\zeta\omega_nt}\left(\cos\omega_n\sqrt{1 - \zeta^2}t + \frac{\zeta}{\sqrt{1 - \zeta^2}}\sin\omega_n\sqrt{1 - \zeta^2}t\right)$
The inverse Laplace transform $C(t)$ of that s-domain $C(s)$ response to a $u(t)$ step