Elastic molecular dynamics with self-consistent flexible constraints

A new algorithm for constrained molecular dynamics is proposed. In contrast to the standard approach, the constrained bond-length/bond-angle value is adjusted at each time step so that total energy is minimized with respect t o the constrained distances. This can be viewed as modifying the equilibriu m bond-length/bond-angle according to external and centripetal forces. Two approaches are constructed to implement the algorithm. Method I includes al l energy terms, but it is neither holonomic nor symplectic. Method II negle cts a rotational kinetic energy term, resulting in a more expensive symplec tic integrator. Both integrators are reversible and well conserve total ene rgy. Due to the iterative nature of the optimization, which requires an ene rgy evaluation at each iteration, these methods are intended for use in con junction with other integration techniques that require an optimization at each time step iteration, such as the self-consistent electronic polarizati on methods or iterative combined quantum mechanics/molecular mechanics (QM/ MM) hybrid simulations. Simulation results for collisions between two diato mic molecules, two water molecules, and for a periodic water box are compar ed and contrasted with fully constrained and free dynamics. These results d emonstrate the utility and efficacy of these new methods. (C) 2000 American Institute of Physics. [S0021-9606(00)50906-4].