Correlations utilising momentum density

The momentum density (partial momentum currents) for each species S,

{\bf M}_{S}(t) = \frac{1}{V}\sum_{n \in S_{i}}m_{n}{\bf v}_{n}(t),

can be calculated in HEATFLUX, for use in thermal-conductivity calculations [1].

These involve correlations and cross correlations of {\bf M}_{S}(t) with the heat flux on a per species basis (less 1 due to conservation).

It should be easy to create these correlations given momentum_density [A B C ...] is in CONTROL.

[1] Cockrell, C., Withington, M., Devereux, H.L., Elena, A.M., Todorov, I.T., Liu, Z.K., Shang, S.L., McCloy, J.S., Bingham, P.A. and Trachenko, K., 2025. Thermal conductivity and thermal diffusivity of molten salts: insights from molecular dynamics simulations and fundamental bounds. The Journal of Physical Chemistry B, 129(8), pp.2271-2279.