The Bees lost 2-0 at Anfield as Luis Diaz and Mohamed Salah struck for the Reds.
Frank said, "A game of two halves. We were very good first half, in a very even game, our pressing, our possession, our defending was good. We tried to get on the ball and create things.
"They had the goal that they scored where we need to do better from a corner. We are this close to getting the first contact on it and where we need to do better, and in the perfect world we need to do better with the two versus two. We must do that.
"We got a free header for Christian Norgaard from a wide free kick, so it was just the small things that split the teams in the first half.
"Second half we couldn't keep the performance up, Liverpool lifted their level, or maybe we could not keep up. Nathan Collins had a good chance before the two nil goal. But overall you can't argue it was a fair and square Liverpool win."
On any differences on facing Arne Slot in comparison with his predecessor at Liverpool, Jurgen Klopp, the Dane added: "There were not many. It is the same players, they have been playing the same way for eight or nine years, not all of them but a lot of them.
"They still have crazy quality all over the pitch. They are still dangerous on offensive transitions, they scored two goals from that.
"There was a few little bits in the structure and the build up play - they were the bigger differences."