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

The longitudinal polarizability (α) and second hyperpolarizability (γ) of the H2 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 α and γ 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. (paper)