How to increase space between rows in begin array equation?

Question

I want to increase space between rows of this vector \overrightarrow{grad} in the equation below:

\begin{eqnarray}
 \nonumber \vec F=\vec P=\left(
 \begin{array}{ccc}
  0\\0\\ _mg
 \end{array}
\right)_{(\vec i, \vec j, \vec k)} \hspace{0.2cm}\textrm{ et }\hspace{0.2cm}
\overrightarrow{grad}V(\sum)=\left(
\begin{array}{ccc}
 \left.\frac{\partial V_x(\sum) }{\partial x}\right.\\ \left.\frac{\partial V_y(\sum) }{\partial y}\right.\\ \left.\frac{\partial V_z(\sum) }{\partial z}\right.
\end{array}
\right)_{(\vec i, \vec j, \vec k)}
\end{eqnarray}

Answer 1

You should never use eqnarray, even less for a single equation.

Also there are some strange things in your input, for instance \left. and \right. that only add (unwanted) spaces and \sum for what should probably be \Sigma.

Better use pmatrix instead of array; I also defined a shorthand for partial derivatives.

For the “touching” fractions, add some vertical space as shown.

\documentclass{article}
\usepackage{amsmath}

\newcommand{\pder}[2]{\frac{\partial #1}{\partial #2}}
\DeclareMathOperator{\grad}{\overrightarrow{\mathrm{grad}}}

\begin{document}

\begin{equation*}
\vec{F}=\vec{P}=
\begin{pmatrix}
0 \\ 0 \\ {}_mg
\end{pmatrix}_{(\vec{i}, \vec{j}, \vec{k})}
\quad\text{et}\quad
\grad V(\Sigma)=
\begin{pmatrix}
\pder{V_x(\Sigma)}{x} \\[1ex]
\pder{V_y(\Sigma)}{y} \\[1ex]
\pder{V_z(\Sigma)}{z}
\end{pmatrix}_{(\vec{i}, \vec{j}, \vec{k})}
\end{equation*}

\end{document}