Center Wide Multiline Equation Tag with ‘aligned’ Environment

Question

I am using aligned environment to write a multiline equation which works out fine if the equation is not too long.

However, this equation is just a bit wider so I want the tag to be still at the middle line (Line 2 in the MWE) which has space. I know the solution can be to squeeze the space between symbols or break the equation even more. But I am curious about how to force the tag position to the center?

Please consider the following MWE:

\documentclass[twocolumn,letterpaper]{article}
\usepackage{amssymb,amsmath,amsfonts,amsthm}
\usepackage{bm}

\begin{document}
  The following equation is a it wider so the tag is squeezed below,
  but there is space on the second line.
  How can I force that tag to that place?
  \begin{equation}
    \begin{aligned}
      \mathrm{vec}(\mathbf{H}_{t,k})=&\ (\mathbf{V}_k^*\otimes\mathbf{V}_M)\,\mathrm{vec}(\bm{\Gamma}_{t,k}\bm{\Omega}_t\bm{\Sigma}_{t,k}) \\
      =&\ (\mathbf{V}_k^*\otimes\mathbf{V}_M)(\bm{\Sigma}_{t,k}^T\otimes\bm{\Gamma}_{t,k})\,\mathrm{vec}(\bm{\Omega}_t) \\
      =&\ (\mathbf{V}_k^*\otimes\mathbf{V}_M)(\bm{\Sigma}_{t,k}^T\otimes\bm{\Gamma}_{t,k})(\mathbf{V}_N^T\odot\mathbf{V}_N)\bm{\Psi}^*
    \end{aligned}
  \end{equation}
\end{document}

which produces this:

Answer 1

A possibility with the \mathrlap command from mathtools and the fleqn environment from nccmath ro ensure the equation starts at the left margin. Note that you don't have to load amsmath in this case, as mathtools does it for you.

\documentclass[twocolumn,letterpaper]{article}
\usepackage{amssymb,mathtools,amsthm}
\usepackage{nccmath}
\usepackage{bm}

\begin{document}

  The following equation is a it wider so the tag is squeezed below,
  but there is space on the second line.
  How can I force that tag to that place?
    \begin{fleqn}
    \begin{equation}
    \begin{aligned}
      \mathrm{vec}(\mathbf{H}_{t,k}) &=\ (\mathbf{V}_k^*\otimes\mathbf{V}_M)\,\mathrm{vec}(\bm{\Gamma}_{t,k}\bm{\Omega}_t\bm{\Sigma}_{t,k}) \\
      &=\ (\mathbf{V}_k^*\otimes\mathbf{V}_M)(\bm{\Sigma}_{t,k}^T\otimes\bm{\Gamma}_{t,k})\,\mathrm{vec}(\bm{\Omega}_t) \\
      &=\ (\mathbf{V}_k^*\otimes\mathbf{V}_M)(\bm{\Sigma}_{t,k}^T\otimes\bm{\Gamma}_{t,k})(\mathbf{V}_N^T\odot \mathrlap{\mathbf{V}_N)\bm{\Psi}^*}
    \end{aligned}
    \end{equation}
    \end{fleqn}

\end{document}