Finite-field many-body perturbation theory IX. Electric properties of ammonia

The complete fourth-order MBPT expansion is applied to the accurate study of the multipole moments and polarizabilities of ammonia by using the finite-field perturbation method and a large partly optimized GTO/CGTO basis set. The results for the dipole and quadrupole moments and the dipole polarizability are in very good agreement with the available experimental data. The accurate theoretical data (μe = 0·5898ea 0, ϑe = -2·210ea 2 0, (α xx )e = 13·73e 2 a 2 0 E -1 h, (α zz )e = 15·66e 2 a 2 0 E -1 h, (Cxx, xx )e = 27·3e 2 a 4 0 E -1 h, (Czz, zz ) e = 30·2e 2 a 4 0 E -1 h, (Cxz, xz )e = 24·0e 2 a 4 0 E -1 h, (Cxy, xy )e = 19·7e 2 a 4 0 E -1 h) are used to evaluate the quality of the interaction surfaces for van der Waals complexes of NH3 computed with a smaller and less flexible basis set. The reasons for some discrepancies between earlier calculations of the dipole polarizability of NH3 are elucidated as well.