Born-Oppenheimer molecular-dynamics simulations of finite systems: Structure and dynamics of (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>O<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

A method for calculations of the ground-state energy and structure of finite systems and for molecular-dynamics simulations of the evolution of the nuclei on the Born-Oppenheimer ground-state electronic potential-energy surface is described. The method is based on local-spin-density functional theory, using nonlocal pseudopotentials and a plane-wave basis set. Evaluations of Hamiltonian matrix elements and the operations on the wave functions are performed using a dual-space representation. The method, which does not involve a supercell, affords accurate efficient simulations of neutral or charged finite systems which possess, or may develop, multipole moments. Since the ground-state electronic energy and the forces on the ions are calculated for each nuclear configuration during a dynammical simulation, a relatively large time step can be used to integrate the classical equations of motion of the nuclei (1--10 fs, depending on the characteristic frequencies of the ionic degrees of freedom). The method is demonstrated via a study of the energetics, structure, and dynamics of the water dimer, (${\mathrm{H}}_{2}$O${)}_{2}$, yielding results in agreement with experimental data and other theoretical calculations. In addition to the properties of the ground state of the dimer, higher-energy transition structures involved in transformations between equivalent structures of the (${\mathrm{H}}_{2}$O${)}_{2}$ molecule, were studied, and finite temperature simulations of the dynamics of such transformations are presented.