Personality differences are a bit tricky.

Each point of personality difference adjusts aspiration by -1.5 to a maximum of -7.5 per characteristic (for a 5+ point difference). This is for each personality characteristic (Neat, Nice, etc.). Each personality characteristic rounds down separately so actually is limited to -7. People with completely opposite personalities (5+ points difference in each of the 5 characteristics) will have -35 attraction.

In addition, if both sims have a characteristic on the same half of the scale (either both above 5 or both below 5) then there is a +7 bonus (which I previously reported in error as +5). So a slightly neat sim (Neat 6) will like very neat sim (Neat 8) [(2 * -1.5) + 7 = +4] more than he would like a slightly sloppy sim (Neat 4) [2 * -1.5 = -3].

Note that if either sim has a 5 in a characteristic then there is no bonus. It's only of both sims are over 5 or under 5.

The bonus is again for each characteristic so sims of exactly the same personality (with none of them at 5) get +5 * 7 = +35 chemistry.

Confusing isn't it.