Molecules in static electric fields:: Linear and nonlinear polarizability of HCN and HCP

Accurate linear and nonlinear polarizabilities were obtained from finite-field self-consistent field, fourth-order many-body perturbation theory and coupled-cluster calculations for the triply bonded linear molecules H-C=N and H-C=P. The mean dipole polarizability and the anisotropy of HCN at the CCSD(T) level of theory is <(alpha)over bar>= 16.74 and Delta alpha= 8.38e(2)a(0)(2)E(H)(-?(1). For HCP the respective values are 35.47 and 16.24 e(2)a(0)(2)E(H)(-?(1). Electron correlation reduces significantly the magnitude of the first dipole hyperpolarizability (beta(alpha beta gamma)) of both molecules. The CCSD(T) values of the mean <(beta)over bar> are [self-consistent-field (SCF) values in parentheses] -2.8 (-7.6) for HCN and 28.5 (36.7) e(3)a(0)(3)E(H)(-?(2) for HCP. Electron correlation modifies mainly the longitudinal component of the second hyperpolarizability tensor gamma(alpha beta gamma delta) for both HCN and HCP. The CCSD(T) mean value for HCN is <(gamma)over bar>= 22.0 x 10(2), 17.4% higher than the SCF value of 18.8 x 10(2)e(4)a(0)(4)E(H)(-?(3). For HCP <(gamma)over bar>= 10.2 x 10(3) at the CCSD(T) level, only 2.2% above the SCF result of 99.8x10(2)e(4)a(0)(4)E(H)(-?(3). For the quadrupole polarizability, fourth-order many-body perturbation theory yields C-ZZ,C-ZZ = 68.58, C-XZ,C-XZ = 40.51, C-XX,C-XX = 34.98 e(2)a(0)(4)E(H)(-)(1) for HCN, and C-ZZ,C-ZZ=202.28, C-XZ,C-XZ=114.44, C-XX,C-XX=106.38 e(2)a(0)(4)E(H)(-?(1) for HCP, with z as the molecular axis.