Some simple systems, with exact values, fail to converge

In doing property-based testing of the optimizer, I've noticed that some simple systems fail to converge.  For example:

```
1	0	0.0
0	1	0.0
1	1	-3.171667e1
```

and

```
2.623620156538e-9
1.075755232314e-4
```

Even slight perturbations of this input will converge properly, for example, changing the energy here to `-3.171167e1`.