To calculate a preflop GTO gameplan requires a lot of computation power, especially compared to a postflop GTO gameplan. The reason is that there are many more possible nodes to calculate preflop than postflop.
Finding a solution requires that all players have a 100% range, with predefined boundary conditions for preflop and postflop. With these fixed conditions, a solving algorithm tries to find the perfect ranges for each player in any possible scenario.
Imagine a player who has AA and is only allowed to use small sizings when postflop. Because AA is a strong hand, we want to increase the pot size postflop. It therefore becomes essential that a big 3Bet/4Bet sizing is allowed preflop. Conversely, if large sizings are only allowed postflop, then AA will played preflop passively as a trap far more often.
Finding the optimal conditions for pre/- and postflop was one of the first significant challenges which Poker Scientist had to solve.
After we discovered the Preflop-Ranges, we simplified them. That means in detail that we removed the hands with an EV of nearly 0 and rounded the frequencies to make them more usable.