Model Hamiltonian

This is written in the form

$$H(t) = H_0 + V(t)$$ (3.1)

with the time-dependence in the perturbation $V(t)$. The case $H(t) = H_0(t) + V$ with a constant perturbation operator but a time-dependent `free part' also exists, but is slightly less used.

The term `time-dependent' perturbation theory, however, primarily refers to perturbation theory for the time-dependence of the wave function and is also used for time-independent Hamiltonians $H(t)=H$.