I am a bit puzzled by strange convergence behavior of DMRG for the transverse field Ising model.

Using the Julia version, I am running the transverse field Ising model example for various system sizes N and the coupling h

```
H = - \sum_{i} Z_i Z_{i+1} - h \sum_{i} X_i
```

My issue is that for certain values of h and N, the convergence seems to become very poor. I set energy tolerance of 1e-12 as a convergence criterion using a custom DMRGObserver, as shown in the documentation. For h>1.0, everything is good and DMRG converges very quickly. For example, even at critical h=1.0, I find it takes around ~10 sweeps.

However, for h=0.5, I notice that the convergence is very fast small N<10, but becomes quite poor for N~15, and then becomes very fast for N>24. For example, in a typical run, I find the following number of sweeps needed to reach an energy tolerance of 1e-12:

```
Row │ N sweeps energy_tol
─────┼───────────────────────────
1 │ 6 7 9.3e-13
2 │ 8 14 7.2e-13
3 │ 10 23 8.2e-13
4 │ 12 55 4.3e-13
5 │ 16 100 2.0e-09
6 │ 24 4 1.4e-14
7 │ 32 4 3.1e-14
8 │ 48 4 3.2e-13
```

Note the rows for N=10,12,16. I am not sure what's happening at these intermediate values of N... why does the convergence suddenly become poor? This behavior depends on the coupling h. For example, in contrast to the above example, at h=1.0 (critical), there seems to be no problem and DMRG converges with very few sweeps (less than 10) within a tolerance of 1e-12 for N=6,..,48.

```
Row │ N sweeps energy_tol
─────┼───────────────────────────
1 │ 6 4 9.7e-16
2 │ 8 6 1.4e-15
3 │ 10 5 8.7e-14
4 │ 12 6 9.5e-16
5 │ 16 6 3.3e-13
6 │ 24 8 9.6e-15
7 │ 32 11 4.9e-13
8 │ 48 12 4.7e-14
```

On the other hand, for h=0.2, I find that even L=6 has problem converging, but larger values of L become much better:

```
Row │ N sweeps energy_tol
─────┼───────────────────────────
1 │ 6 100 2.5e-09
2 │ 8 36 6.9e-13
3 │ 10 3 2.6e-13
4 │ 12 4 1.1e-14
5 │ 16 3 4.0e-13
6 │ 24 4 5.5e-15
7 │ 32 4 1.5e-15
8 │ 48 4 5.4e-15
```

Larger h (greater than 1.0) seem to be all well behaved when I did a few spot checks. Does this behaviour make sense? My initial state is just a randomMPS. I tried playing with sweeps, both including noise and without. But that doesn't seem to matter very much. It's counter-intuitive to me that the problem is with small lattices, while larger ones do okay. Can you see what could be happening here? Thanks!