Natural orbital functional calculations of molecular polarizabilities and second hyperpolarizabilities. The hydrogen molecule as a test case

The longitudinal polarizability (alpha) and second hyperpolarizability (gamma) of the H-2 molecule as a function of the internuclear distance are calculated using the PNOF5 level of theory. It is shown that PNOF5 gives accurate results for the longitudinal alpha and gamma over the whole curve range, even for those structures exhibiting a high degree of diradical character. The good agreement between these results and reference full-CI data highlights the adequacy of PNOF5 to treat electronic systems exhibiting large static electron correlation and shows a promising accuracy in calculating nonlinear optical properties for systems covering a large range of diradical character. The results further support the paradigm of using molecules with intermediate diradical character to enhance the second hyperpolarizabilities.