Dear All,
I am trying to compute the lattice constant of copper at different temperatures > 0K.
I am using fix NPT.
I measure the volume change and then calculate the lattice constant.
But, I am unable to find an equilibrated lattice constant.
Does anyone have any suggestions on finding lattice constant at different temperatures?

Try using the minimize command with fix box/relax. You can include
a kinetic component in the computed pressure. You can also
do it with fix npt but will need to equilibrate carefully, since small
volume changes produce large pressure changes in a solid.
Steve