ACCURATE ELECTRIC MULTIPOLE MOMENTS FOR HCN AND HCP FROM CCSD(T) CALCULATIONS WITH LARGE GAUSSIAN-BASIS SETS

Accurate values of the electric multipole moments of HCN and HCP have been obtained from self-consistent field (SCF) and coupled-cluster (CC SD(T)) calculations. With the origin at the centre of mass and hydroge n along the positive molecular axis in both systems, a [9s5p2d/10s7p5d 3f/10s7p5d3f] basis set is expected to predict near-Hartree-Fock value s for the dipole (mu = 1.2962ea(0)), quadrupole (Theta = 2.1046ea(0)(2 )), octopole (Omega = 10.088ea(0)(3)) and the hexadecapole (Phi = 24.2 3ea(0)(4)) moment of HCN. An analogous basis set, [9s5p2d/10s7p5d3f/ 1 4s11p7d3f], predicts SCF values of mu = 0.1421ea(0), Theta = 3.8786ea( 0)(2), Omega = 19.633ea(0)(3) and Phi = 65.89ea(0)(4) for HCP, Electro n correlation reduces the dipole moment of HCN but increases the dipol e moment of HCP. At the CCSD(T) level of theory the calculated values are mu = 1.1800ea(0), Theta = 1.6461ea(0)(2), Omega = 9.762ea(0)(3) an d Phi = 22.45ea(0)(4) for HCN and mu = 0.1710ea(0), Theta = 3.2312ea(0 )(2), Omega = 16.578ea(0)(3) and Phi = 60.87ea(0)(4) for HCP.