Knowledge Bank

An accurate full dimensional methane potential energy surface (PES) will aid in assigning and understanding its complicated spectrum. Heretofore, there is no pure ab initio PES of methane on which rovibrational levels have errors less than one cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. % In this work, we obtain an accurate methane PES by starting with the ab initio PES of Schwenke and Partridge [Spectrochim. Acta A {\bf 57}, 887 (2001)] and adjusting 5 of their parameters to reproduce 39 reliable vibrational levels of CH$4$. This reduces the rmsd from 4.3 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ to 0.4 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. %which include all vibrational levels up to the Octad polyad %and only 4 levels of the Tetradecad polyad Since not all of the Tetradecad levels are certain, only 4, those confirmed by direct experimental transitions, are included in the fit. The new PES ought therefore to aid in the ongoing analysis of the Tetradecad polyad. To further test the accuracy of the new PES, vibrational and rovibrational levels are computed for CH$4$, CH$3$D, CHD$3$ and CH$2$D$2$ and are compared with the extensive experimental data. The errors are all within about one cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. % %\vspace{1em} % \noindent The fitting is made possible by a contracted-iterative method, 101 (2003).} for computing vibrational levels in a product of contracted stretch and bend functions. The fitting process is efficient because these contracted basis functions are not changed during the fitting cycles, which greatly reduces the time (to about 3 hours) to compute a new set of vibrational levels when the PES is slightly changed.