The propagation method for imaginary-time evolution.
Options:
it_forward_euler:
Approximate the evolution operator by one forward Euler step with the exponential:
$
U_{\rm FE}(t+\delta t, t) = \exp \left( -\tau H_{t}\right)\,.
$
This scheme is first order.
it_expmid:
Similar to forward Euler, but evaluate the Hamiltonian at the half-timestep by
extrapolation:
$
U_{\rm EM}(t+\delta t, t) = \exp \left( -\tau H_{t+\delta t/2}\right)\,.
$
This scheme is second order.