  If an open quantum system is initially uncorrelated from its environment,
then its dynamics can be written in terms of a Lindblad-form master equation.
The master equation is divided into a unitary piece, represented by an
effective Hamiltonian, and a dissipative piece, represented by a
hermiticity-preserving superoperator; however, the division of open system
dynamics into unitary and dissipative pieces is non-unique. For
finite-dimensional quantum systems, we resolve this non-uniqueness by
specifying a norm on the space of dissipative superoperators and defining the
canonical Hamiltonian to be the one whose dissipator is minimal. We show that
the canonical Hamiltonian thus defined is equivalent to the Hamiltonian
initially defined by Lindblad, and that it is uniquely specified by requiring
the dissipator's jump operators to be traceless, extending a uniqueness result
known previously in the special case of Markovian master equations. For a
system weakly coupled to its environment, we give a recursive formula for
computing the canonical effective Hamiltonian to arbitrary orders in
perturbation theory, which we can think of as a perturbative scheme for
renormalizing the system's bare Hamiltonian.
