SYSTEMATIC STUDIES OF MOLECULAR VIBRATIONAL ANHARMONICITY AND VIBRATION-ROTATION INTERACTION BY SELF-CONSISTENT-FIELD HIGHER DERIVATIVE METHODS: APPLICATIONS TO ASYMMETRIC AND SYMMETRIC TOP AND LINEAR POLYATOMIC MOLECULE

Inclusion of the anharmonicity of molecular normal mode vibrations (i.e., the third and fourth (and higher) derivatives of a molecular Born-Oppenheimer potential energy surface) is necessary in order to theoretically reproduce experimental fundamental vibrational frequencies of a molecule. Although ab initio determinations of harmonic vibrational frequencies may give errors of only a few percent by the inclusion of electron correlation within a large basis set for small molecules, in general, molecular fundamental vibrational frequencies are more often available from high resolution vibration-rotation spectra. Recently developed analytic third derivatives methods for self-consistent-field (SCF) wavefunctions have made it possible to examine with previously unavailable accuracy and computational efficiency the anharmonic force fields of small molecules. In particular, cubic force constants, and quartic force constants by finite differences of cubic force constants, allow theoretical determination of a number of anharmonic molecular properties, including vibration-rotation interaction constants, vibrational anharmonic constants, fundamental vibrational frequencies, quartic and sextic centrifugal distortion constants, and rotational constants which include zero-point vibrational and centrifugal distortion corrections, and vibrational and rotational $ell$-type doubling constants and rotational $ell$-type doubling constants.;Application is made here to a variety of asymmetric and symmetric top and linear polyatomic molecules in order to predict their anharmonic properties. Quadratic, cubic, and quartic force constants are evaluated for the molecules H$sb2$O, H$sb2$S, H$sb2$CO, HCO($sp2$A$spprime$), CH$sb2$($sp3$B$sb1$), CH$sb2$($sp1$A$sb1$), CH$sb2$($sp1$B$sb1$), C$sb2$H$sb4$, HCN, CO$sb2$, N$sb2$O, COS, C$sb2$H$sb2$, H$sbsp{3}{+}$, NH$sb3$, and several isotopomers. For most molecules the anharmonic molecular constants which are available from experiments are well reproduced theoretically using DZP or better basis sets, at which level the calculated constants seem to have converged with respect to basis set expansion as well, although exceptions have been noted. Particularly good agreement is found for fundamental vibrational frequencies obtained from CISD harmonic frequencies and SCF anharmonic corrections within the same basis set.