P119, Mukamel's book "Principles of Nonlinear Optical Spectroscopy"
Eq.(5.21) in Liouville space
$$ S^{(2)}(t_2,t_1)=\left(\frac{i}{\hbar}\right)^2 \left\langle \left\langle V\left|\mathscr{G}(t_2)\mathscr{V}\mathscr{G}(t_1)\mathscr{V}\right|\rho(-\infty) \right\rangle\right\rangle $$
in...