Depth for UCC ansatz

Question

I tried to understand the notion of depth vs Trotter step in a uccsd circuit for the vqe algorithm in qiskit (I use an older version, 0.15). When I tried understanding it for a small system like H2, and started changing depth from 1 through 3, the whole circuit for U (the variational form, which in this case is uccsd) started repeating itself. Physically, this looked very much like increasing Trotter steps (or num_time_slices), for example, e^(A+B) = (e^(A/3)e^(B/3))^3. Indeed, when I set num_time_slices as 2 and depth as 1, I got the same circuit as with the numbers switched. Can someone help me with how the two are different, viz, depth and Trotter step?

Answer 1

It is a bit strange that they decided to include "depth" as a parameter for UCCSD circuit. As you noted, the depth just means the repetition of the circuit/var_form here. So in fact, we do not need it as we can increase the number of time slices. Also note that in the new Qiskit release, "depth" is not called "reps" which, obviously, stands for repetition .

Also note that UCCSD is a chemically motivated Ansatz, so it only makes sense that the starting state, "initial_state" start out as the Hartree-Fock state.