What would the best way to typeset HI to designate molecular gas as in the title of this article, with a capital H and a slightly smaller numeral I ? I am using the scrbook class.

I tried to define

\newcommand{\HI}{H {\uppercase{\romannumeral 1}} }

but this results in H i, with a lowercase i.

I also tried


but this doesn't behave properly when I use it in \section{}, resulting in "H i" rather than "H I".

I also haven't found anything like this in chemmacros, which is more geared towards chemists.

This is a standard way of writing molecules in astrophysics, so I'm surprised there isn't a package that does this easily.

  • it's a tex generated file on arxiv, can't you just download the tex source and see, there is no need to use \uppercase and \romannumeral they would just generate a normal I anyway so you could simply use HI Mar 8, 2019 at 9:24
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    perhaps \textsc{Hi} Mar 8, 2019 at 9:29
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    I would not recommand the way it seems in this article... The distance between H and I makes it difficult in reading as one "thing" and possibly the author would try to use another way of defining a command in future edition of this article. @DavidCarlisle's above comment would give a better result I suppose.
    – koleygr
    Mar 8, 2019 at 9:42
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    The article you linked to uses H\,{\footnotesize I} - however I don't think this is the best way to go (you can download the tex source of the article from arxiv.org/format/1608.02601) Mar 11, 2019 at 10:55
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    Personally I would use the \ion macro from the "Astronomy & Astrophysics" journal class: fits.gsfc.nasa.gov/standard30/aa.cls Mar 11, 2019 at 12:55

3 Answers 3


Use the size of subscripts linked to the current font size.


  \mbox{H\check@mathfonts\fontsize\sf@size\z@\selectfont I}%

\textheight=3cm % just to shorten the image


\section{About \HI}

Here we talk about \HI.\footnote{Yes, about \HI.}


enter image description here

The kernel command \check@mathfonts computes \sf@size and \ssf@size starting from the current font size, either with the parameters stored with \DeclareMathSizes or with a heuristic otherwise.

  • The spacing is off when HI is in the middle of a sentence. \HI is cool ends up in the document as HIis cool.
    – usernumber
    Jun 5, 2019 at 17:27
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    @usernumber That's a well known fact; just add {} at the end: \HI{}. It is discussed in tex.stackexchange.com/q/31091/4427, but adding {} is the most effective way, notwithstanding what others suggest.
    – egreg
    Jun 5, 2019 at 17:29

Adjust 1.2ex to suit.


\section{This is \HI}

And in text: \HI{} and \LARGE \HI

enter image description here

Because scalerel operates, by default, in math mode, the $I$ is actually an escape out of math mode. If that is too confusing, you could always just operate in math mode as: \newcommand\HI{H\protect\scaleto{\textrm{I}}{1.2ex}}

If you wanted a version without packages, you could do this, but it requires a separate macro for bold style:


\section{This is \HIbold}

And in text: \HI{} and \LARGE \HI

enter image description here


Another way to do it is the way it is implemented by the Astronomy & Astrophysics class.

Add this declaration to the header :


Then in the body of the document, use


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