What Is the Relationship between Theta and Gamma for Short-Dated ATM Options?

For short-dated ATM options, the relationship between Theta and Gamma is highly positive and pronounced. These options have the highest Gamma (rapid Delta change) and the highest Theta (rapid time decay).

This means the option is highly sensitive to both price movement and time passing. The high positive correlation is a key feature of short-dated ATM options, reflecting their dual role as high-risk/high-reward instruments.