# mhchem ver.4 not working after update! Control sequence undefined

I updated MikTex 2.9 Monday, and now can't compile while loading 'mhchem'. I'm using pdflatex [Version 3.14159265-2.6-1.40.16 (MiKTeX 2.9 64-bit)] to compile the following:

\documentclass{article}
\usepackage[version=4]{mhchem}
\begin{document}
Hello, world!
\end{document}


throws the following error:

! LaTeX error: "kernel/command-not-defined"

! Control sequence \prop_get:Nn undefined.

! See the LaTeX3 documentation for further information.

! For immediate help type H <return>.

l.180 ..._variant:Nn \prop_get:Nn           { cV }

| This is a coding error.

| LaTeX has been asked to use a control sequence '\prop_get:Nn': this has not been defined yet.

• It runs without error in texlive updated just now. add \listfiles to the preamble and post the resulting list from the end of the log file. – David Carlisle Mar 8 '16 at 16:25
• Can't compile, throws 100 errors, so the file list is not shown! – TobiasDK Mar 8 '16 at 16:31
• This is the log file: db.tt/1GGf9vFa – TobiasDK Mar 8 '16 at 16:33

You have a local old copy of chemgreek masking your installed one:

(C:\Users\TobiasGram\AppData\Roaming\MiKTeX\2.9\tex\latex\chemgreek\chemgreek.sty


^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

("C:\Program Files\MiKTeX 2.9\tex\latex\l3packages\xparse\xparse.sty"
Package: xparse 2016/01/19 v6377 L3 Experimental document command parser
\l__xparse_current_arg_int=\count170
\l__xparse_m_args_int=\count171
\l__xparse_mandatory_args_int=\count172
\l__xparse_processor_int=\count173
\l__xparse_v_nesting_int=\count174
)
Package: chemgreek 2015/04/09 v1.0 interface for upright greek letters for use in chemistry (CN)


It should be more like

(/usr/local/texlive/2015/texmf-dist/tex/latex/chemgreek/chemgreek.sty
(/home/davidc/texmf/tex/latex/l3packages/xparse/xparse.sty
Package: xparse 2016/01/19 v6377 L3 Experimental document command parser
\l__xparse_current_arg_int=\count170
\l__xparse_m_args_int=\count171
\l__xparse_mandatory_args_int=\count172
\l__xparse_processor_int=\count173
\l__xparse_v_nesting_int=\count174
)
Package: chemgreek 2016/02/10 v1.0e interfaceforuprightgreeklettersforuseinchem
istry (CN)

• I don't use miktex but I gather from other answers seen here that it has user and admin installations and you need to update both, probably. – David Carlisle Mar 8 '16 at 16:41
• If I just delete it, should that be okay, and then update it again? – TobiasDK Mar 8 '16 at 16:42
• I don't know how miktex's package manager reacts to deleting things by hand, just updating your installation under Users\TobiasGram\AppData\Roaming should be enough, or wait for someone who's used miktex to pass by and confirm:-) – David Carlisle Mar 8 '16 at 16:55
• I uninstalled 'chemgreek' from Packages Manager, and deleted the 'chemgreek folder in AppData, and reinstalled 'chemgreek' again. Then Refresh FNDB and Update Formats in Settings. Working now :-) Thank you for the help David! – TobiasDK Mar 8 '16 at 17:20
• If the chemgreek packages is vers. 1.0d, then one need to do a manuel install from CTAN: mirrors.ctan.org/macros/latex/contrib/chemgreek.zip – TobiasDK Mar 11 '16 at 12:53