NLO responses of small polymethineimine oligomers: A CCSD(T) study

With the aim to decrease the uncertainties in the so far predicted values, the static longitudinal linear and non-linear optical properties of small trans-transoid polymethineimine oligomers have been obtained at the CCSD(T) level using the 6-311++G(3df,3pd) basis set and MP2-optimized ground state geometries. Our results are compared with previous theoretical data obtained at lower levels of theory and/or using less accurate geometries. In addition, the second hyperpolarizability is reported for the first time. Electron correlation effects are analyzed at various levels of the theory, including MP2, CCSD and CCSD(T). It turns out that (i) CCSD(T) systematically decreases dipole moment values (reduced per unit cell) by 4-5%, (ii) a previously published MP2 extrapolated polymeric dipole polarizability value of 126.1 a.u. should be considered as an upper bound, and (iii) our CCSD(T) results on the first hyperpolarizability lead to a significantly smaller estimation (by about 60%) of the polymeric value.