yes, that formula is for equilibrium MD simulations in NVT.
About your system, you can adjust the density by running MD in NPT ensemble first at standard pressure. Then, after system is equilibrated. You can fix your V at the average value you get from MD in NPT, and switch to NVT ensemble.
You can see these references, they also discuss calculation of heat capacity from md simulations. It seems that classical md simulations overestimate it, but quantum corrections may be an improvement. You can also see results from different water models.