High order dipole moment surfaces of PH3 and ab initio intensity predictions in the Octad range

New dipole moment surfaces (DMS) of phosphine are constructed using extended ab initio CCSD(T) (CCSD(T)-F12) calculations at 11 697 nuclear configurations. The DMS analytical representation is determined through an expansion in symmetry adapted products of internal nonlinear coordinates up to the 8th order. Including high order terms permits significantly reducing the fit errors for ab initio dipole moment values in a large range of nuclear geometries. Rovibrational line strengths in the infrared are variationally computed from DMS using normal mode Eckart frame approach. Comparison of results obtained with four basis sets up to V5Z is presented. Integrated intensities of four lower PH 3 polyads up to J = 20 are in a good agreement with the HITRAN 2012 database and with available spectroscopic data analyses. A particular focus of the study is the Octad range (2650–3700 cm −1 ) containing eight interacting bands 3ν 2 , 2ν 2 + ν 4 , ν 2 + 2ν 4 , ν 1 + ν 2 , ν 2 + ν 3 , 3ν 4 , ν 3 + ν 4 and ν 1 + ν 4 , where new ab initio predictions are expected to help further analyses of experimental spectra.