The largest number of adjacent tiles
...
You want the capital to be surrounded by as many blank (bonus-less) tiles as possible.
Except that, as mentioned earlier, this is quite clearly a subobtimal choice of location for the colony capital except in cases where you'd only be building a small number of buildings which benefit one another in the area. Three mutually adjacent factories is 6 bonus levels; two factories adjacent to one another and to the the colony capital is merely 4 bonus levels. Two factories which are both adjacent to a third but not adjacent to one another is 4 bonus levels; the same configuration with the colony capital occupying the center slot is only two bonus levels, while the same configuration with the colony capital occupying one of the end tiles is three bonus levels. Why exactly do I want my colony capital adjacent to as many tiles as possible?
The only situation I see where it's beneficial for me to have as many tiles adjacent to the colony capital as possible is the situation where I'm building relatively few structures which benefit from being next to one another in an area, such as the earlier example of the class-7 world which had six tiles adjacent to a central tile. Under any other circumstance, the colony capital is just not worth having in the cluster; I can always improve the cluster by shifting the colony capital out of it and replacing it with a building that would benefit from the other structures in the cluster.
I began to speculate "what if colony capitals could take advantage of adjacency bonuses" which led me to wonder what a colony capital surrounded by 6 antimatter plants would look like.
Depends on how they handled the adjacency bonuses that the colony capital would receive. Most structures give a level bonus related to the purpose of the structure; if this were true of colony capitals, I would tend to expect to see a bonus to population (it's a bit less complicated than giving +X% to manufacturing, wealth, and research output, and a bonus to maximum population is, to some degree, equivalent; a bonus to population growth would be similar in the early stages of colony development but would become less useful later in the colony's life). A model which, to me, would be more interesting would be if the bonuses a colony capital received from its levels was dependent upon the where the bonuses came from, but I don't think that's currently something which the game can do.
