The general impact model for trading a VWAP order throughout the day has the form of:
$\alpha \cdot \sigma_n \cdot \text{(participation rate)}^\beta$
I'm looking for an impact / slippage model of any form for the open and closing crossing acution for US equities (or any other assetclass).
What would you use instead of $\sigma_n$, and what would you set as a pessimistic estimate or upper bound on $\alpha$ and $\beta$?
I prefer simulations to be careful and pessimistic, therefore any upper bound on these quantitites or even a rule of thumb would help.