Application of the Floquet theory in calculations of higher-order nonlinear optical susceptibilities of molecules

The results of calculations of the nonlinear optical susceptibilities are analyzed, which were performed up to the fourth order inclusively, according to the algorithm suggested earlier by the authors and based on the combination of formulas of the perturbation theory with the finite field method for quasi-energy states. The results obtained for the helium atom and the lithium hydrate molecule are compared to the values of the same quantities calculated by the methods of the nonstationary theory. A comparatively small discrepancy, which does not exceed 25%, allows one to consider this method as the basis for ab initio calculations of nonlinear optical processes of higher orders.