The Metropolis-Adjusted Langevin Algorithm (MALA), originally introduced to
sample exactly the invariant measure of certain stochastic differential
equations (SDE) on infinitely long time intervals, can also be used to
approximate pathwise the solution of these SDEs on finite time intervals.
However, when applied to an SDE with a nonglobally Lipschitz drift coefficient,
the algorithm may not have a spectral gap even when the SDE does. This paper
reconciles MALA's lack of a spectral gap with its ergodicity to the invariant
measure of the SDE and finite time accuracy.
This paper presents a counterexample to the conjecture that the semi-explicit
Lie-Newmark algorithm is variational. As a consequence the Lie-Newmark method
is not well-suited for long-time simulation of rigid body-type mechanical
systems. The counterexample consists of a rigid body in a static potential
field.