Steering single neurons in a molecule-generating model changes one property while staying valid

alex_peys · x · 2026-10-30

Inspired by the viral Qwen single-neuron steering experiments, alexpeys tested whether the ideas transfer to autoregressive models that speak a non-human language—sending Codex to hunt for neurons in a decoder-only model trained to generate molecules.

It worked: single neurons, when steered, keep the model generating valid molecules but change just one property—for example, one neuron produces molecules with more rings. Chart data: mean ring count rose from 2.87 to 3.57 (unconditioned) and 2.79 to 3.58 (conditioned), 1,024 steered outputs per arm per condition, with logP/TPSA conditioning bounds still satisfied.

Takeaway: mech interp techniques work beyond natural language, on molecular SMILES too—"maybe mech interp is fun."

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