A SYMPLECTIC METHOD FOR RIGID-BODY MOLECULAR SIMULATION

Rigid-body molecular dynamics simulations typically are performed in a quaternion representation. The nonseparable form of the Hamiltonian i n quaternions prevents the use of a standard leapfrog (Verlet) integra tor, so nonsymplectic Runge-Kutta, multistep, or extrapolation methods are generally used, This is unfortunate since symplectic methods like Verlet exhibit superior energy conservation in long-time integrations . In this article, we describe an alternative method, which we call RS HAKE (for rotation-SHAKE), in which the entire rotation matrix is evol ved (using the scheme of McLachlan and Scovel [J. Nonlin. Sci, 16 233 (1995)]) in tandem with the particle positions. We employ a fast appro ximate Newton solver to preserve the orthogonality of the rotation mat rix. We test our method on a system of soft-sphere dipoles and compare with quaternion evolution using a 4th-order predictor-corrector integ rator, Although the short-time error of the quaternion algorithm is sm aller for fixed time step than that for RSHAKE, the quaternion scheme exhibits an energy drift which is not observed in simulations with RSH AKE, hence a fixed energy tolerance can be achieved by using a larger time step, The superiority of RSHAKE increases with system size. (C) 1 997 American Institute of Physics.