Connected triple excitations in coupled-cluster calculations of hyperpolarizabilities: neon

We have calculated the second hyperpolarizability γ of neon using the CCSD(T) method. The accuracy of the CCSD(T) approach has been established by explicit comparison with the single, double and triple excitation coupled-cluster (CCSDT) method using extended basis sets that are known to be adequate for the description of γ. Our best estimate for γ0 of 110±3 au is in good agreement with other recent theoretical values and with Shelton's recent experimental estimate of 108±2 au. Comparison of the MP2 and CCSD(T) hyperpolarizability values indicates that MP2 gives a very good description of the electron correlation contribution to γ0. We have combined MP2 frequency-dependent corrections with the CCSD(T) γ0 value to yield γ(-2ω; ω, ω, 0) and γK(-ω; ω, 0, 0). © 1992.