  An extended Hamiltonian approach to conduct isothermal-isobaric molecular
dynamics simulations with full cell flexibility is presented. The components of
the metric tensor are used as the fictitious degrees of freedom for the cell,
thus avoiding the problem of spurious cell rotations and artificial symmetry
breaking effects present in the original Parrinello-Rahman scheme. This is
complemented by the Nose-Poincare approach for isothermal sampling. The
combination of these two approaches leads to equations of motion that are
Hamiltonian in structure, and which can therefore be solved numerically using
recently developed powerful symplectic integrators. One such integrator, the
generalised leap-frog, is employed to provide a numerical algorithm for
integrating the isothermal-isobaric equations of motion obtained.
