[Tex/LaTex] Equation number is not in same line in two-column document class

Question

 \documentclass[aps,twocolumn]{revtex4}
 \usepackage{epsfig}
  \usepackage{amsmath}
  \usepackage{epsfig}
   \renewcommand{\baselinestretch}{1.45} 
 \begin{document}
 \begin{equation}
  V_S(x)=V_1 \Theta(x/a) + iV_2 \delta(x/a),~   
   \Theta(x<0)=0,\Theta(x>0)=1.    
  \end{equation}
 \end{document}

Equation number is not coming on same line, in two column document class, please help ??

Answer 1

The material in the equation environment simply doesn't fit in a single row, so amsmath is forced to place the equation number one row lower.

To keep the display-math material from exceeding the column width, you'll have to introduce a line break somewhere. The following screenshot provides two possible solutions (the horizontal lines are there just to illustrate the width of the columns):

 \documentclass[aps,twocolumn]{revtex4}
 \usepackage{amsmath} 
 \begin{document}
 \hrule
      \begin{multline}
      V_S(x)=V_1 \Theta(x/a) + iV_2 \delta(x/a),\\ 
         \Theta(x<0)=0,\quad\Theta(x>0)=1.    
      \end{multline}
 \hrule
 \bigskip\noindent
 Or:
 \begin{equation}
  V_S(x)=V_1 \Theta(x/a) + iV_2 \delta(x/a)
  \end{equation}
  where
  \begin{align*}
     \Theta(x<0)&=0,\\
     \Theta(x>0)&=1.    
  \end{align*}
 \hrule
 \end{document}