A new method of molecular dynamic computer simulation at constant temperature T and pressure P has been proposed. Unlike the earlier techniques of this sort, the new approach does not assume the volume V of the crystal to be an independent variable but the one that can be estimated from the coordinates of the atoms in the crystal. The new method was carefully tested on a finite hcp crystal whose Lennard-Jones potential of the two-atom interaction is known. The test proved the efficiency of the new method under different conditions including T=0 and P>0.