How to make a new tex command in org-mode-latex?

Question

In a general tex-file, one can easily define a new command, for my case, let me take this as an example:

To put this code in header

\newcommand{\dd}{\,\mathrm{d}}

I can use the shorter command to make the longer command work; it's good to do this, I also can compile org-file into tex if this command is added in header. However the preview C-c-x-l fails to recognize \dd even if this new command have been defined in header.

I think we can add some code in org package to make this work, right? how can I do this? I want to make preview C-c-xl recognize \dd as \mathrm{d}.

Update

Let me expound my question.

I hope we can amend or add some codes in ~/org path/lisp/org-latex.el to make preview recognize \dd as \,\matherm{d} in every org-file.

The way @pmr posed works in each org-file which added that code by hand. I expect we can make this as default and needn't to add such code every time.

Answer 1

You need to add your newcommands to the header

#+LaTeX_HEADER: \newcommand{\dd}{\,\mathrm{d}}

* TestOrg
  $\dd  d$

then they are considered by preview.

Let this be the place where I rant: Don't do it. This will cage your documents inside LaTeX and you will never get them out again without some significant effort. No matter how much people tell you to use newcommand to define your won aliases: don't touch it.