In molecular dynamics (MD) simulations the need often arises to maintain such parameters
as temperature or pressure rather than energy and volume, or to impose gradients for
studying transport properties in nonequilibrium MD. A method is described to realize
coupling to an external bath with constant temperature or pressure with adjustable time
constants for the coupling. The method is easily extendable to other variables and to
gradients, and can be applied also to polyatomic molecules involving internal constraints …