# Equation number is not in same line in two-column document class

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


• Welcome to TeX.SE. Your equation is too wide for the column. Are you willing to introduce a line-break somewhere in the equation? – Mico Sep 21 '17 at 5:39
• @Mico By reducing the size, it can be fitted to same line , could you please tell me how to fit in same line ?? – math Sep 21 '17 at 5:57
• By making the font size smaller, you may indeed achieve the objective of making the material fit in a single line -- at the considerable cost of making the material illegible and all but guaranteeing that your readers won't bother to figure out what you're trying to say. If some material is important enough for you to want to show it in your paper, its content shouldn't be carelessly deprecated by rendering the material in a very small font. – Mico Sep 21 '17 at 5:58

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),\\
$$V_S(x)=V_1 \Theta(x/a) + iV_2 \delta(x/a)$$