Can an energy drink or chocolate before an exam be useful ? What sort of other edible goods be helpful?

I think doing such things may be risky. Instead, do something natural. The most important is to get plenty of sleep the night before the exam and don't try to cram up to the last minute.

Between the two exams parts, get some light exercise and, again, don't cram. Let your mind relax and let it integrate what you know rather than trying to force it to absorb things that may mask other, more important things.

As for food, eat as you normally do but nothing heavy that will make you groggy and wanting an afternoon nap. If you eat differently, your gut may rebel against you.

If you have been taking good notes throughout the learning period, then summarizing them on note cards (big ideas only) in the week previous can be a good plan. You can then do a light review at the morning and mid-day meal. A light review. Big ideas.

But make your body and mind as comfortable as possible. Breathe deeply. Relax your face. Smile. (These are martial arts advices, actually.)

Whether it has positive, negative ot neutral effects is completely up to the person. You may do some tries before - drink/don't drink and do some concentration tasks, compare your performances. About question 2, another things which could have positive or negative are drinking coffee, doing sport and learning new things just before the exam. (For most people, the last one is bad.)

Of course, you should not take any "strong" substances which need a doctor's prescription without discussing this with a doctor. After all, it's just an exam - your health is worth so much more!

I had the experience a couple years ago of conducting a hearing. There were ten days of hearing and each day the hearing went from 9 to 5 with an hour break for lunch. Each day was as intense as an all-day exam. I found that my concentration in the afternoon was best if I followed the diet that is recommended for gestational diabetes: frequent small meals that emphasize protein. I had a light breakfast and then a snack around 11. The lunch break started at 12:30 or 1, and I found it helpful to make a point of only eating half of my usual lunch. Then I had the rest of my lunch a couple of hours later. Following this pattern was extremely helpful.

Proponents of the low-carb diet say that it can be beneficial for everyone, not just those with impaired glucose metabolism, in that it boosts afternoon concentration and productivity.