Accurate higher electric multipole moments for carbon monoxide

We have obtained accurate values for the higher electric multipole moments of CO. We rely on finite-field coupled cluster calculations with large gaussian-type basis sets. At the CCSD(T) level of theory the quadrupole moment varies as [Θ(R)−Θ(Re)]/ea02=1.03(R−Re)+0.40(R−Re)2 around the experimental bond length of Re=2.132221 a0. For the octopole and hexadecapole moment we find [Ω(R)−Ω(Re)]/ea03=1.49(R−Re)−1.81(R−Re)2−0.45(R−Re)3−0.17(R−Re)4, [Φ(R)−Φ(Re)]/ea04=−4.61(R−Re)+0.67(R−Re)2+2.29(R−Re)3+1.04(R−Re)4. Our best values Ω=3.46 ea03 and Φ=−9.07 ea04 are in reasonable agreement with recent experimental estimates deduced from far-infrared absorption in the high-density gas spectrum of CO–Ar. Agreement is less satisfactory with values obtained from the liquid spectrum.