Dipole hyperpolarizability surfaces of ammonia

Full-dimensional first dipole hyperpolarizability surfaces of NH ₃ are determined by means of multiconfigurational quadratic response theory. On the basis of the two-dimensional, symmetric stretching vs inversion, hyperpolarizability functions, and approximate vibrational wave functions, effective inversional hyperpolarizability moments are evaluated for the D _3h ammonia isotopomers. The calculations have revealed their strong vibrational and frequency dependence. A fairly close reproduction of the first hyperpolarizability ESHG constant β (632.8 nm) by theory indicates that the major correction to the one-particle hyperpolarizability is obtained by variational wave functions including a comparatively small set of active orbitals. We find that a meaningful comparison with experimental results can only be obtained by an appropriate evaluation of both the vibrational and the correlated dispersion dependencies.