Knowledge Bank

{Ab initio} calculations on methanol structural and fundamental vibrational frequency changes occuring with internal rotation of the methyl group were carried out with completely relaxed geometry optimizations using second-order M\o ller-Please perturbation theory (MP2) in the Gaussian 94 package on the Cray YMP-90. The changes due to the methyl group internal rotation were carefully examined in a detailed study of the angle dependence with the internal rotation angle monitored every $10\circ$ for a complete $360\circ$ rotation. The purpose of work is to try to get calculated values from Gaussian 94 for the various parameters (especially coefficients of variatuions with internal rotation angle) determined in our global fits of microwave and far-infrared methanol data. This will see if commercial {ab initio} packages are now accurate enough to be helpful for internal rotation problems. Work is just getting under way at the time of writing, and some problems have arisen (which may be oerator error). We are thus trying to determine why 1. At the high level of accuracy with which we are concerned, the {ab initio} calculation results are sensitive to the Z-matrix chosen. 2. All our results on structural and fundamental vibrational frequency changes preserve the plane of symmetry for methanol (symmetry operation: $\gamma \to \gamma \pm \pi )$, whereas not all the results show the three fold symmetry expected for methanol (symmetry operations: $\gamma \to \gamma \pm 2\pi /3$ and $\gamma \to \gamma \pm 4\pi /3$). We note that our energies and structures at the torsional minimum (internal rotation angle $\gamma =\pi$) and maximum (internal rotation angle $\gamma =0$) are consistent with the literature {ab initio} $calculationa$. We hope to overcome these problems and plan to present some {ab initio} results for comparison with our methanol global fit parameters.