Accurate electric multipole moments for HCN and HCP from CCSD(T) calculations with large Gaussian basis sets

Summary Accurate values of the electric multipole moments of HCN and HCP have been obtained from self-consistent field (SCF) and coupled-cluster (CCSD(T)) calculations. With the origin at the centre of mass and hydrogen along the positive molecular axis in both systems, a [9s5p2d/10s7p5d3f/10s7p5d3f] basis set is expected to predict near-Hartree-Fock values for the dipole (=1.2962ea₀), quadrupole (=2.1046ea₀²), octopole (=10.088ea₀³) and the hexadecapole (=24.23ea₀⁴) moment of HCN. An analogous basis set, [9s5p2d/10s7p5d3f/14s11p7d3f], predicts SCF values of =0.1421ea₀, =3.8786ea₀², =19.633ea₀³ and =65.89ea₀⁴ for HCP. Electron correlation reduces the dipole moment of HCN but increases the dipole moment of HCP. At the CCSD(T) level of theory the calculated values are =1.1800ea₀, =1.6461ea₀², =9.762ea₀³ and =22.45ea₀⁴ for HCN and =0.1710ea₀, =3.2312ea₀², =16.578ea₀³ and =60.87ea₀⁴ for HCP.