MOLECULES IN STATIC ELECTRIC-FIELDS - LINEAR AND NONLINEAR POLARIZABILITY OF HC-N AND HC-P

Accurate linear and nonlinear polarizabilities were obtained from fini te-field self-consistent field, fourth-order many-body perturbation th eory and coupled-cluster calculations for the triply bonded linear mol ecules H-C=N and H-C=P. The mean dipole polarizability and the anisotr opy 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 val ues are 35.47 and 16.24 e(2)a(0)(2)E(H)(-?(1). Electron correlation re duces significantly the magnitude of the first dipole hyperpolarizabil ity (beta(alpha beta gamma)) of both molecules. The CCSD(T) values of the mean <(beta)over bar> are [self-consistent-field (SCF) values in p arentheses] -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 compone nt of the second hyperpolarizability tensor gamma(alpha beta gamma del ta) for both HCN and HCP. The CCSD(T) mean value for HCN is <(gamma)ov er 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 a s the molecular axis.