in the same directions, and leaving their relative angle unchanged. []{data-label="Fig1.1"}](Fig1.pdf){width="49.00000%"}

The modification of the Onsager excluded volume interactions among hard rods due to self propulsion is worked out in Ref. [@Baskaran2010]. Here we simply give a qualitative description of this effect and we refer the reader to that work for the technical details. First we note that the presence of a self propulsion speed along the long axis of the nematogen, results in a breaking of the nematic symmetry of the the microdynamics, as shown in Fig. \[Fig1\]. On the other hand, since the interactions conserve momentum, this cannot lead to a macroscopic breaking of polar symmetry as this would amount to the appearance of a spontaneous macroscopic momentum from a zero momentum state. Hence, only a homogeneous ordered nematic state can occur and the associated mean field transition will be the same as in the case of the active nematic considered above, albeit with coefficients $
\alpha$ and $\beta$ renormalized by self-propulsion [@Baskaran2008a]. Even though the polar symmetry cannot be broken macroscopically, momentum conservation allows the nematic ordering to induce local polar order in the system. To illustrate this, let us consider hard rods in two dimensions undergoing 