Typography of atomic orbital subscripts p_x

It the notation $\mathrm{p\_x}$, $\mathrm{p\_y}$ and $\mathrm{d\_{x^2{-}y^2}}$, are the subscripts variables (and therefore should be in italics) or are they labels (labels for directions, I suppose, and therefore should be upright)?




The subscripts that specify orbital axes are written in *italic*. This is mentioned in ACS style guide.$^{[1]}$ So for instance


$$\mathrm{d}\_{x^2 - y^2}.$$


Be careful though. Other subscripts for orbitals are generally upright, *e.g.*,


$$\mathrm{t\_{2g}}.$$


[$[1]$](http://dx.doi.org/10.1021/bk-2006-STYG) Anne M. Coghill, Lorrin R. Garson. ($2006$). *The ACS Style Guide. Effective Communication of Scientific Information*. American Chemical Society. **DOI**: 10.1021/bk-2006-STYG, **ISBN**: 9780841239999 (print), 9780841228306 (online). (p 256)


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