Single (C–C) and triple (CC) bond-length dependence of the static electric polarizability and hyperpolarizability of H–CC–CC–H

We report an ab initio study of the static electric (hyper)polarizability of diacetylene and its dependence on the single (C–C) and triple (CC) bond length. At the CCSD(T) level of theory we find for the mean dipole polarizability and its derivatives ᾱ=49.10 e2a02Eh−1,(∂ᾱ/∂RC−C)e=−4.41 and (∂ᾱ/∂RCC)e=34.57 e2a0Eh−1. For the anisotropy Δα=54.45 e2a02Eh−1, (∂Δα/∂RC–C)e=−20.42 and (∂Δα/∂RCC)e=64.56 e2a0Eh−1. The dependence of the mean hyperpolarizability on RC–C and RCC around the equilibrium is quite distinct. Varying the single bond by ΔR/a0 around the equilibrium entails changes of[γ̄(RC–C)−γ̄(Re)]/e4a04Eh−3=−3643ΔR−230ΔR2−184ΔR3+453ΔR4The mean second hyperpolarizability increases strongly with RC≡C around the equilibrium[γ̄(RCC)−γ̄(Re)]/e4a04Eh−3=22 259ΔR+11 293ΔR2+2384ΔR3+6445ΔR4