The behaviour of a real gas in usually depicted by plotting compression factor $\left(Z\right)$ versus p at a constant temperature. At low temperature and low pressure, Z is usually less than one. This fact can be explained by van der Waals equation when 

The electrical polarisation that is generated in each particle by the presence of other particles, which results in relatively mild attractive forces acting on neutral atoms and molecules.

On neglecting b in comparison to $V_{\mathrm{m}}$ in van der Waals equation, we get

$\left(p+\frac{a}{V_{\mathrm{m}}^2}\right)\left(V_{\mathrm{m}}\right)=RT$ 

or $p{V}_{\mathrm{m}}=RT-a/V_m$ 

or $Z=\frac{p{V}_{\mathrm{m}}}{RT}=1-\frac{a}{V_{m}RT}$