In the methane molecule, CH_4, each hydrogen atom is at a corner of a regular tetrahedron with the carbon atom at the center. In coordinates where one of the {rm C} {bf- } {rm H} bonds is in the direction of hat{ i } + hat{ j } + hat{k}, an adjacent {rm C} {bf- } {rm H} bond is in the hat{ i } – hat{ j } – hat{k} direction