Knowledge Bank

The geometries, harmonic frequencies, and force fields of $CF4$ and $SiF4$ have been determined by use of ab initio molecular orbital theory with the STO-3G, 4-31G, and $6-31G\ast$ basis sets. The effect of inclusion of correlation has been considered using M{\o}ller Plesset theory to second order. Since sufficient experimental data are available for $CF4$ and $SiF4$ to determine the actual harmonic frequencies, these calculations provide an interesting test of the predictability of frequencies from ab initio theory. The differences between the calculated and experimentally derived harmonic frequencies for $CF4$ and $SiF4$ are similar to the differences found for smaller molecules for which harmonic frequencies are available. The theoretical force field is compared to force fields obtained from vibrational analyses of $CF4$ and $SiF4$ frequencies. Limitations of ab initio force fields for higher Z systems such as $SiF4$ are discussed.