Inconsistent FoM with use_FFT True and False

**Description of the error**
There is some inconsistency between the results with "use_FFT": False and "use_FFT": True. Here are three runs with "use_FFT": True

```
Step FoM CMA itera... epsilon (#2) sigma (#3)
0 0.979 1 1.08 3.4

Step FoM CMA itera... epsilon (#2) sigma (#3)
0 0.6 1 1.08 3.4

Step FoM CMA itera... epsilon (#2) sigma (#3)
0 0.9683 1 1.08 3.4
```

and three runs with "use_FFT": False

```
Step FoM CMA itera... epsilon (#2) sigma (#3)
0 47.47 1 1.08 3.4

Step FoM CMA itera... epsilon (#2) sigma (#3)
0 48.04 1 1.08 3.4

Step FoM CMA itera... epsilon (#2) sigma (#3)
0 47.06 1 1.08 3.4
```

**Describe the expected result**
FoM should be similar.

**Additional details**
Here I plotted the argon results with epsilon=1.08, sigma=3.40; surprisingly, they are similar, so the FFT code seems to be working, but it causes the FoM to be different.