BORN-OPPENHEIMER MOLECULAR-DYNAMICS SIMULATIONS OF FINITE SYSTEMS - STRUCTURE AND DYNAMICS OF (H2O)2

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, (H2O)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 (H2O)2 molecule, were studied, and finite temperature simulations of the dynamics of such transformations are presented.