You don't want \textbf
for vectors, but \vec
. “Wait!” you'll say, “I don't want vectors with an arrow above their head!”
Don't worry! Neither do I!
What I mean is that you should use a semantic command for your objects and \vec
is a good choice.
The main problem in your code is \textbf{v\vdot\textbf{r}}
: \textbf
starts text mode, and \vdot
is only valid in math mode. Anyway, also
\mathbf{v\vdot\mathbf{r}}
is wrong and should be \mathbf{v}\vdot\mathbf{r}
. Better yet, use \vec
(or other semantic command named as you prefer) like below.
\documentclass[12pt,a4paper]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{bm}% for the second choice
\usepackage{physics}
\renewcommand{\vec}[1]{\mathbf{#1}}
%\renewcommand{\vec}[1]{\bm{#1}}
\begin{document}
\begin{equation}
\Gamma_{\vec{P}}(\vec{r})=
-\frac{2\pi}{m}\int\frac{\dd[3]{k}}{(2\pi)^3} e^{-i\vec{k}\vdot\vec{r}}f_p(\vec{k})
\end{equation}
\end{document}
By the way, a subscript should be braced, if it doesn't consist in a single letter or number (but bracing also these doesn't harm). Also align
should not be used for single displayed equations.

Should you change your mind and decide that bold italic for vectors is better, just switch the comment sign %
and enable
\renewcommand{\vec}[1]{\bm{#1}}
to get, upon rerunning LaTeX,

\documentclass{...}
, the required\usepackage
's,\begin{document}
, and\end{document}
. That may seem tedious to you, but think of the extra work it represents for TeX.SX users willing to give you a hand. Help them help you: remove that one hurdle between you and a solution to your problem. – Moriambar May 17 '17 at 20:42