Nonlinear optical property calculations of polyynes with long-range corrected hybrid exchange-correlation functionals

Polarizabilities (alpha), second-hyperpolarizabilities (gamma), and the gamma scaling factors (c) of polyynes [H-(C[triple bond]C)(n)-H, n = 1-8] were evaluated using the long-range corrected (LC) density functional theory (DFT) and LC-DFT with a short-range Gaussian attenuation (LCgau), as well as high quality wavefunction methods. We show that the c values obtained from LC- and LCgau-DFT are consistent with those from CCSD(T) calculations. Furthermore, the polyyne c values we obtained are seen to be smaller than the c values derived from previously calculated polyene gamma values [Sekino et al., J. Chem. Phys. 126, 014107 (2007)] in all the methods we consider. We compare our results with those obtained experimentally [Shepkov et al., J. Chem. Phys. 120, 6807 (2004).] from end-capped polyynes [i-Pr(3)Si-(C[triple bond]C)(n)-Sii-Pr(3)], which show larger c values for polyynes than polyenes. Our alpha and gamma calculations with i-Pr(3)Si-(C[triple bond]C)(n)-Sii-Pr(3) (n = 4, 5, 6, and 8) show that i-Pr(3)Si- may participate in pi molecular orbital delocalization, which can unexpectedly affect the c value. We also confirm the importance of molecular geometry in these nonlinear optical calculations. We find that while LC- and LCgau-DFT excellently reproduce experimental geometries and bond length alternation (BLA), MP2 optimized geometries have a BLA that is too short to be used for accurate alpha and gamma calculations.