Knowledge Bank

Self-consistent-field (SCF) analytic third derivatives with respect to nuclear perturbations for both $closed1$ and $open2$ shell molecules can now be routinely calculated for molecules of spectroscopic interest. In this talk, two such studies will be reported. Recently, the interactions of the stretching and bending motions of the isolated CH chromophores in $CF3H,CClF2H,CCl3H$, and $CO3H$ have attracted much experimental $attention3$. We have evaluated the SCF cubic force field analytically for these systems using a variety of cartesian gaussian basis sets. The interpretation of both the experimental and theoretical data will be discussed. The best fit of experimetal data to obtain an anharmonic force field for $CH4$ has been performed by Gray and $Robiette4$. In their analysis, a single quartic force constant ($frrrr$) and full cubic force field were utilized. We have calculated the complete ab initio quartic force field. Fourth derivatives were obtained from finite differences of analytic third derivatives. The resulting ab initio anharmonic constants will be presented and discussed. Finally, recent progress in the analytic evaluation of SCF fourth derivatives will be presented.