# Eliminate huge-locant-marked (by \overset) atom side space

Is it possible to elegantly eliminate side whitespace around the base under the overset, which is longer than the base?

E.g. if we wanted to talk about the 123456789th carbon atom of a huge poly(methylene) (or polyethylene, if you want) molecule, and mark the discussed carbon atom locant, like:

\ce{CH3-[CH2]_x-\overset{123456789}{C}H2-[CH2]_y-CH3} …


we get

$$\ce{CH3-[CH2]_x-\overset{123456789}{C}H2-[CH2]_y-CH3} ~~~ (x=123456789-2, \; x \le y)$$

resulting rendering image follows, for convenience

We could fix it with some negative spaces (\!)

… \overset{\!\!\!\!\!\!\!\!\!\!\!123456789\!\!\!\!\!\!\!\!\!\!\!}{C}H2 …


$$\ce{CH3-[CH2]_x-\overset{\!\!\!\!\!\!\!\!\!\!\!123456789\!\!\!\!\!\!\!\!\!\!\!}{C}H2-[CH2]_y-CH3} ~~~ (x=123456789-2, \; x \le y)$$

which is inexact (and, for me, works in Cr but not in FF browser).

Is it possible to fix the spacing problem a better way?

Probably not, at least probably not in the way you would like it. As far as I see there is no centering zero-width box implemented in MathJax (in LaTeX one could use makebox with \makebox[width][pos]{text}), there are right/left overlapping boxes though. I guess the following is a (mediocre) workaround, but it's the best I could come up with.

$$\ce{CH3-[CH2]_x-\overset{123456789}{C}H2-[CH2]_y-CH3}$$
$$\ce{CH3-[CH2]_x-\overset{\rlap{_\swarrow123456789}}{C}H2-[CH2]_y-CH3}$$
$$\ce{CH3-[CH2]_x-\overset{\llap{123456789_\searrow}}{C}H2-[CH2]_y-CH3}$$


$$\ce{CH3-[CH2]_x-\overset{123456789}{C}H2-[CH2]_y-CH3}$$ $$\ce{CH3-[CH2]_x-\overset{\rlap{_\swarrow123456789}}{C}H2-[CH2]_y-CH3}$$ $$\ce{CH3-[CH2]_x-\overset{\llap{123456789_\searrow}}{C}H2-[CH2]_y-CH3}$$

With MathJax there are limited options; it is not LaTeX. I don't know why you need this, but it might be better to just completely avoid such constructs.
MathJax tends to be slow, too, so better avoid anything too complicated. $$\odot\!\frown\!\odot$$

If you want to know which (La)TeX commands are supported, head on over to Dr. Carol JVF Burns overview.