Unit normalisations
All equations in Legolas are in dimensionless form, as is common practice when dealing with (M)HD.
As usual we have three degrees of freedom: the unit magnetic field $B_u$ and unit length $L_u$ should be set, together with either the unit density $\rho_u$ OR unit temperature $T_u$.
Mean molecular weight
Unit normalisations depend on the molecular weight $\bar{\mu}$, and in Legolas we usually distinguish between two cases:

Pure proton plasma: $\bar{\mu} = 1$, this is the default case.
\[\bar{\mu} = \dfrac{m_i n_i}{n_i} = m_i \rightarrow \bar{\mu} = 1\] 
Electronproton plasma: $\bar{\mu} = 0.5$, in this case the molecular weight should be explicitly set.
\[\bar{\mu} = \dfrac{m_e n_e + m_i n_i}{n_e + n_i} \simeq \dfrac{m_i n_i}{n_e + n_i} = \dfrac{1}{2}m_i \rightarrow \bar{\mu} = \dfrac{1}{2}\]
Normalisations
We have two options: either the unit density is set or the unit temperature is set. In both cases a unit magneticfield and a unit length should be set as well, so the unit pressure is given by
\[p_u = \dfrac{B_u^2}{\mu_0},\]taking a plasmabeta reference value of 2. The magnetic constant is given by $\mu_0$ ($= 4\pi$ in cgs).
We then have the following options:

If $\rho_u$ chosen along with $B_u$ and $L_u$:
\[T_u = \dfrac{\bar{\mu}m_p p_u}{k_B \rho_u},\] 
If $T_u$ chosen along with $B_u$ and $L_u$:
\[\rho_u = \dfrac{\bar{\mu}m_p p_u}{k_B T_u},\]
All other normalisations follow from these, where the Alfvén speed is assumed as reference velocity:
\[\begin{gather} m_u = \rho_u L_u, \\ n_u = \dfrac{\rho_u}{m_p}, \\ v_u = \dfrac{B_u}{\sqrt{\mu_0 \rho_u}}, \\ t_u = \dfrac{L_u}{v_u}, \\ \Lambda_u = \dfrac{p}{t_u n_u^2}, \\ \kappa_u = \dfrac{\rho_u L_u v_u^3}{T_u}, \end{gather}\]which, from top to bottom, read as mass unit, numberdensity unit, velocity unit, time unit, cooling curve unit and conduction unit. The proton mass is given by $m_p$ and $k_B$ denotes the Boltzmann constant.
Note: The unit normalisations are only relevant when radiative cooling, thermal conduction or temperaturedependent resistivity is included. We always set base values though (as one should), which are given in cgs units by $B_u = 10$ G, $L_u = 10^9$ cm and $T_u = 10^6$ K. If no normalisations are specified these are used by default.