Because there is no barostat, so there is no pressure scaling matrix, therefore atomic positions are not scaled and box vectors are invariant.

Because there is no barostat, so there is no pressure scaling matrix, therefore atomic positions are not scaled and box vectors are invariant.

As the name of the ensemble NVT is saying, the Number of particles, Volume and the Temperature are constant. The volume is constant just be freezing the simulation box (the lattice constant and the angle are fixed). [Actually, constant volume is not equivalent with fixed simulation box, but frozen simulation box is a common approach.] The thermostat controls only the distribution of the kinetic energy to the various freedom degrees, and does not act on the simulation box.

I agree with Justin and Viorel. You need a NPT ensemble (isotropic or semi-isotropic, it depends on your system), as Justin said (it means the use of a barostat).