Writing equations within dgroup environment

Question

I am having a problem with writing equations using dmath and dgroup in my article. the packages I am using are

\documentclass[a4paper 12pt] {article}
\usepackage{breqn}

The codes I am writing are -

\begin{dgroup}
\begin{dmath}
v_f = k_f C_O(0,t) = \frac{i_c}{nFA}
\end{dmath}
\begin{dmath}
v_b = k_b C_R(0,t) = \frac{i_a}{nFA}
\end{dmath}
\label{eq14}
\end{dgroup}

The problem is that it has two equal signs in the same line of equation. with this settings I am getting the results okay but after the two lines of equations, LaTeX puts a big gap before I start my text again. When I use equation in place of dmath, the big space after the two lines of equations are gone but the equations are not at two lines any more. It puts the = \frac{i_c}{nFA} part into the next line, which I don't want. Can anyone please help me with a solution?

Answer 1

A relation symbol that shouldn't be considered an alignment point in dmath should be marked with \hiderel:

\documentclass[a4paper,12pt] {article}
\usepackage{breqn}

\begin{document}

\begin{dgroup}
\begin{dmath}
v_f = k_f C_O(0,t) \hiderel{=} \frac{i_c}{nFA}
\end{dmath}
\begin{dmath}
v_b = k_b C_R(0,t) \hiderel{=} \frac{i_a}{nFA}
\end{dmath}
\label{eq14}
\end{dgroup}
\end{document}

I really can't see much advantage than writing

\documentclass[a4paper,12pt] {article}
\usepackage{amsmath}

\begin{document}

\begin{subequations}\label{eq14}
\begin{align}
v_f &= k_f C_O(0,t) = \frac{i_c}{nFA}
\\
v_b &= k_b C_R(0,t) \hiderel{=} \frac{i_a}{nFA}
\end{align}
\end{subequations}

\end{document}

Answer 2

It's always worth posting complete documents. The space after a display depends on lots of things in the example below it is slightly larger gap to line 2 after the breqn layout than to line 7 after the equation layout., but not really noticeably so, but perhaps you were comparing to the gap before line 4 with two consecutive equations, that is noticeably less as the \belowdisplayshortskip is used in that case (which is 6pt rather than 10pt here).

\documentclass{article}
\usepackage{breqn}
\showoutput
\begin{document}

1Abc def ghi jkl mno pqr stu vwx yz
\begin{dgroup}
\begin{dmath}
v_f = k_f C_O(0,t) = \frac{i_c}{nFA}
\end{dmath}
\begin{dmath}
v_b = k_b C_R(0,t) = \frac{i_a}{nFA}
\end{dmath}
\label{eq14}
\end{dgroup}
2Abc def ghi jkl mno pqr stu vwx yz

3Abc def ghi jkl mno pqr stu vwx yz
\begin{equation}
v_f = k_f C_O(0,t) = \frac{i_c}{nFA}
\end{equation}
\begin{equation}
v_b = k_b C_R(0,t) = \frac{i_a}{nFA}
\label{eq14b}
\end{equation}
4Abc def ghi jkl mno pqr stu vwx yz


5Abc def ghi jkl mno pqr stu vwx yz
\begin{equation}
v_f = k_f C_O(0,t) = \frac{i_c}{nFA}
\end{equation}
6Abc def ghi jkl mno pqr stu vwx yz
\begin{equation}
v_b = k_b C_R(0,t) = \frac{i_a}{nFA}
\label{eq14c}
\end{equation}
7Abc def ghi jkl mno pqr stu vwx yz

\end{document}