The whole report cannot even compile now! And the pdf cannot open any more! I narrowed down the problem to \usepackage{mhchem}. I was using version 3 (if it matters) but i also tried writing version=4. I read here that there is an issue when updating latex. So i tried uninstalling and installing the previous version that was working and still nothing. An example:


        \ce{C$H_{1.4}O_{0.4}$ -> 1''C$H_{2}$ '' + 0.4H2O}

It does nothing. trying to finish compile or just say:

Undefined control sequence. \begin{document}.

  • 1
    (1) welcome, (2) why are you using those $ in math mode? And what does _{1.4} of hydrogen actually mean.
    – daleif
    Nov 28, 2016 at 16:03
  • 1
    On a recent TeX installation, I don't get the error "undefined control sequence" but complaints from the mhchem package about characters that cannot be interpreted. The complaints go away when I remove the $ signs (math mode is not required for subscripts to work) as well as the quotes ''.
    – gernot
    Nov 28, 2016 at 16:06

1 Answer 1


You get an error Unexpected input character for the '. They are not supported (yet). But in your example, I don't even know what they should represent. What is 1''?

By the way, this input definitely did never work with any version of mhchem.

You don't need the $, just remove them.

\ce{CH_{1.4}O_{0.4} -> 1CH2 + 0.4H2O} works just fine. (Although 2+0.4*2=2.8, not 1.4.)

Unfortunately, up to v4.04 (which seems to be included in TeX Live 2016, from reports I get) there was a bug, causing an infinite loop of Unexpected input character when running in nonstop mode, as is the case when compiling within TeX editors.


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