Equation not displayed in latex - latex

Question

The equation below does not display in latex. Also, the introductory text does not display properly(all words together without space and in italics)
A common form of this potential is the 12-6 Lennard Jones (LJ) potential
expressed as equation 2.7
\begin{align}
U_{ij}= 4\epsilon_{ij}[(\frac{\sigma_{ij}}{r_{ij})^12-
(\frac{\sigma_{ij}}{r_{ij})^6]
\end{align}
Also, in the text below, all words are together without space and in italics.
Where \epsilon_{ij} and \sigma{ij} represent well depth and diameter of
the atom respectively. \epsilon_{ij} and \sigma_{ij} for unlike atoms
are determined using Lorentz-Berthelot combination rules [44] given in
equations 2.8 and 2.9
Your help is appreciated.
I attach here a picture of the output.

Answer 1

This is because you're not using inline math mode properly. In the image you posted
it shows that your text is preceded by (what looks like) \epsilon_{ij}. \epsilon requires a math font, and therefore you should use $\epsilon_{ij}$. The same goes for any inline math you want to typeset. This would be the suggested/proper coding:
A common form of this potential is the 12-6 Lennard Jones (LJ) potential
expressed as equation~\eqref{eq:lj-potential},
\begin{equation}
U_{ij} = 4 \epsilon_{ij} [ (\frac{\sigma_{ij}}{r_{ij})^{12}
- (\frac{\sigma_{ij}}{r_{ij})^6 ] \label{eq:lj-potential}
\end{equation}
where $\epsilon_{ij}$ and $\sigma{ij}$ represent well depth and diameter of
the atom respectively. $\epsilon_{ij}$ and $\sigma_{ij}$ for unlike atoms
are determined using Lorentz-Berthelot combination rules~\cite{lorentz-berthelot}
given in equations~\eqref{eq:epsilon} and~\eqref{eq:sigma}.
Note the following:
Use equation for a single-line numbered equation; align is for multi-line equations (that may require alignment).
No blank line (paragraph break) before equation.
Use \labels and \refs (or \eqref, since you're using amsmath) since equation numbers can change. Let TeX take care of storing and recalling these numbers.
Use \cite to reference something in a bibliography.
Use $...$ (or \(...\)) for inline math (already discussed above).

Answer 2

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
U_{ij}= 4\epsilon_{ij}[
(\frac{
\sigma_{ij}
}{
(r_{ij})^{12} -
\frac{
\sigma_{ij}
}{(r_{ij})^6
}
}]
\end{align}
\end{document}
three groups were not closed properly in the equation.
Read about amsmath package.
I suggest you try to write as clean as possible.
Output