Chemistry

CHEMISTRY

NPC autonomy for Fallout 4. Adult NPCs occasionally start AAF scenes with each other, because they chose to — not because a perk was sprayed on them.

Chemistry decides. Rapport does.

▼ CHEMISTRY — decides WHO, WHEN and WHY

rarity · privacy · whose home it is · who keeps saying no

▼ RAPPORT — decides HOW IT GOES, and cleans up after

tree selection · faces · sweat · aftermath · actor locking · cooldowns · repair

▼ AAF — plays the animation, applies overlays and morphs

▼ ENGINE — MFG morphs · LooksMenu overlays · worn geometry

One Papyrus script and one generated ESP, and that is the whole mod. Every pass, Rapport publishes the pairs it can see along with the raw measurements behind its own ranking. Chemistry reads that list and either asks for one scene, or explains in the log why it asked for none.

⚠ One thing to know before you play

Don't fast travel in the moment two NPCs are walking towards each other to start a scene. The game can crash.

When a scene is about to happen, AAF walks the two NPCs together first. That takes roughly ten seconds, and it happens before anything visible starts. Fast travel during those ten seconds and the game may close itself.

• []Once a scene is actually playing, fast travel is fine. Tested repeatedly, no crashes.[/]

  []It is only the few seconds while they are pairing up.[/]

  []If you see two NPCs walking oddly towards each other, wait a moment before you travel.[/]

  This is AAF, and no mod can fix it from the outside — the crash happens inside AAF's own Flash interface while the game is unloading the area under it. Fallout 4 gives mods no way to know a fast travel is about to happen, so there is nothing to hook.

  For the curious — how we know

  Seven controlled trials, each a fresh game:

  • []Fast travel during a running scene: 2 trials, 0 crashes.[/]

    []Fast travel during the pairing-up window: 4 trials, 3 crashes.[/]

  Every crash was an access violation inside Scaleform's ActionScript VM, at null or near-null addresses — AAF's UI state being freed while it was still being used.

  We tried to fix it and the measurements said no. The obvious idea was to stop pairing NPCs who are far apart, so there is less walking. But the window is about 8. 4 seconds even for two people standing on top of each other — the walking is only a small part of it. There is no distance setting that removes this.

  What it is actually trying to be

  • []Rare, and per actor. Twenty-four game hours per actor, not per pair. Per-pair alone would let one settler have a scene with B, then C, then D in an evening — which reads as a mod rather than as people.[/]

    []No global rate, no quota. A busy settlement produces more than an empty one because it contains more pairs. That is the intended shape, not a number someone tuned.[/]

    []Couples can emerge. A pair with history is mildly likelier — and capped, because the failure it guards against is a rut where the same two monopolise a settlement.[/]

    []Whose place it is matters. Their own home earns the long, unhurried story whatever the room count. Somebody walking through your house is not an audience in a market.[/]

    []A crowd is prohibitive; two onlookers are nothing. Deliberately the same tolerance Rapport uses — one layer calling a pair private while the other calls it crowded is a contradiction this shipped with once.[/]

    []It stops asking the NPC who always says no. An escalating backoff on refusals. AAF can leave a busy flag on an NPC for the rest of a save if a request dies badly, and one such NPC would otherwise score top of the list and burn a request every single pass.[/]

    Everything that goes into the decision

    Two layers, and they measure different things. Rapport measures what it can see from C++ every pass; Chemistry adds the policy that only Papyrus can reach.

    First, who is even eligible. Anyone failing these is never a candidate, and the log counts every rejection by reason: unloaded · a child · dead · in combat · out of range · race not on the allow-list · in dialogue · chatting · busy on a quest.

    Then Rapport scores every surviving pair:

    [*]How close they already are — weight 1. 5, falling off with the square of distance. Standing together beats merely sharing a faction; without the