Knowledge Bank

The interaction between molecular hydrogen and other molecules is studied through {\it ab initio} calculations with a focus on differences in behavior between {\it ortho} and {\it para} hydrogen. These differences manifest themselves in various ways, including in the rotational spectra of complexes}, J. Chem. Phys. {\bf 127}, 054305 (2007).}$\mathrm{<mrow\; data-mjx-texclass="ORD">,</mrow>}$}, J. Chem. Phys. {\bf 115}, 5155 (2001).} and in pressure broadening cross sections.}, 4069 (2000).} Two specific systems, H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$-HCN and H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$-OCS, will be examined. For H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$-HCN, a new 4-D potential surface will be presented along with bound state and scattering calculations, while for H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$-OCS new calculations utilizing a previously presented potential} will be discussed. These two systems display markedly different ground state geometries that lead to different behavior of the {\it ortho} and {\it para} hydrogen complexes. In addition to the specific systems a more general discussion will examine the effect of potential anisotropy and ground state geometry on the properties of hydrogen complexes and how the differences between {\it ortho} and {\it para} interactions can be used.