The Bachelier model is a model of an asset price under Brownian motion presented by Louis Bachelier on his PhD thesis The Theory of Speculation (Théorie de la spéculation, published 1900).
The (much) later Black-Scholes-(Merton) Model addresses that issue by positing stock prices as following a log-normal distribution which does not allow negative values.
On April 8, 2020, the CME Group posted the note CME Clearing Plan to Address the Potential of a Negative Underlying in Certain Energy Options Contracts,[1] saying that after a threshold on price, it would change its standard energy options model from one based on Geometric Brownian Motion and the Black–Scholes model to the Bachelier model.
On April 20, 2020, oil futures reached negative values for the first time in history,[2] where Bachelier model took an important role in option pricing and risk management.
[3] The implied volatility under the Bachelier model can be obtained by an accurate numerical approximation.