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General Products hulls are almost indestructible, the only known weakness being antimatter or attacking the powerplant that re-enforces them. This is supposedly because the hull is a single giant molecule.

How exactly does being a single molecule make the hull invulnerable? Why can it not be deformed or damaged by virtue of being a single molecule?

There's little or no real science involved. We already make materials that are all one molecule for all practical purposes (breaking them involves breaking bonds within molecules, not merely separating intact molecules) and while they are very strong by ordinary standards, those materials are nothing like GP hulls.

The most detailed discussion of a GP hull is in the story "Flatlander" where Beowulf Schaeffer and B/r/u/c/e/ P/e/l/z/ Gregory Pelton are asked to explore a mystery planet by the Outsiders. As they approach the planet, they talk about the GP hull: "Bey, can anything get through a General Products Hull?" "Light, like a laser beam. Gravity, like tides crushing you into the nose of a ship when you get too close to a neutron star. Impact won't harm the hull, but it'll kill what's inside." (In another story it's mentioned that the only light that passes through the hull is that used for vision by one of races the Puppeteers sell GP hulls to.)

As they approach the planet, the GP hull suddenly disintegrates. They limp back to civilization and General Products explains: The planet was made of anti-matter, and "A General Products hull is an artificially generated molecule with inter-atomic bonds artificially strengthened by a small power plant. The strengthened molecular bonds are proof against any kind of impact, and heat into the hundreds of thousands of degrees. But when enough of the atoms had been obliterated by antimatter explosions, the molecule naturally fell apart."

So there you have it: Instead of the hull being made of handwavium, it is ordinary matter that is turned into artificial unobtanium by means of a small power plant. There's really nothing more you can say about the technology. OTOH, given the magic, you can speak about the consequences.

GP hull material can't be deformed -- it's very, very hard -- because the individual bonds are very, very strong, yielding an extremely rigid material. Deformation of any sort begins with some bonds being stretched and others being compressed. If the bonds are not deformable or breakable, the material becomes enormously rigid and hard and you get the effect mention in the story: An impact pushes the whole hull without breaking it and the contents thud at high speed against the invulnerable hull and are crushed.

The hull being impervious to electromagnetic radiation other than light is much harder to understand. There are two ways to exclude EMR: reflection and absorption. To reflect EMR, the hull would need either to be conductive -- metals reflect light well, and mirrors have a metal coating -- or have a carefully constructed impedance mismatch. The latter is hard to do across wide frequency ranges and it's not obvious how the GP hull would do this well without it having something else going on than just the artificially strengthened bonds.

Absorption means that the EMR's photons interact with the substance's electrons and energize them. That's one of the things that the GP hull's magic power plant must prevent, so it's hard to see how absorption would stop EMR.

So the artificial rigidity of the bonds (which are made of electrons, after all) which does a nice job of giving the hull rigidity, would also appear to make it transparent to most frequencies of EM radiation! (Even using the SF physics of the story.)

Because the intramolecular force (the force holding atoms in a molecule together) is typically far stronger than the intermolecular force (the force holding molecules in a substance together) - mostly because, in cases where it isn't, the atoms typically rearrange themselves via a chemical reaction until they are, or until there aren't any molecules left (like salt dissolving in water as ions).

This is part of why diamond (a single molecule) is stronger than coal (multiple molecules). It is easier to separate molecules from each other than it is to separate the atoms within the molecules. (And even harder to separate the quarks within the atoms...)

Obviously, this is not the only thing that needs to be taken into account - graphite has stronger intramolecular bonds than diamond, but forms smaller molecules, and has weaker intermolecular bonds. This means that a lump of graphite is quite weak, but a single flawless sheet or tube is extremely strong (i.e. graphene or carbon nanotubes), and over time (lots of time) diamonds will turn into graphite at Standard Temperature & Pressure.

At high pressure and high temperature, diamond is more stable, so graphene will turn into diamond. Presumably, this is the purpose of the powerplant reinforcing the hull: It maintains an environment in which the monomolecular GP hull is the most chemically stable configuration, and damaging it allows the hull to revert to a weaker-but-more-stable state.

To add to the other answers, in the "Fleet of Worlds" series of books it's revealed that the hulls contain generators (hidden within within the seemingly transparent structure) that reinforce the molecular bonds. It never really explains how, it's implied that it's stronger than normal single molecules, it's essentially magitec beyond that.

From Juggler Of Worlds:

"I have finally learned that our hulls can be destroyed without antimatter." Baedeker straightened, hoping to project more confidence than he felt. "Nessus, your alien scouts learned too much about how we build the hulls." Because you told them. "But let us learn from that fiasco."

"Recall that the hull is a single super molecule, the normal interatomic bonds reinforced with energy from an embedded power plant. Shut down the power plant and simple air pressure will blow apart the hull."

"I thought the power plant was sealed," Achilles said.

Baedeker bobbed heads. "True, the embedded power plant has no external controls. But since the hulls are transparent, laser signaling can turn them off."

Vesta whistled with confusion. "Surely not. Why embed an off command?"

"Humans are very good with math and computers." Nessus seemed to be thinking aloud. With a shiver, he returned his attention to the conference. "There is no off switch, but there are embedded processors. Optical computers, embedded in transparent hull material. That's it, Nike. Obviously they found a way to optically hack into the controls and shut them down."

As I recall from discussion of General Products hulls in various stories and books (Neutron Star and various short stories, then the Ringworld series), the in-universe explanation given by Nessus the Puppeteer in Ringworld (many years after the hulls were discontinued and GP, as a business, folded -- coincident with the onset of the Puppeteer worlds' flight from the Core Explosion) is that the hull is a monomolecule, similar in concept to the very dangerous Sinclair Monofilament (which can cut through nearly anything by virtue of being a single atom thick), with the interatomic bonds augmented by a small, unobtrusive, and self-powered force field generator.

The known properties of General Products hulls are: transparent to all wavelengths used for vision by any of GP's customer species, and opaque to all other radiation, indestructible unless too much of the hull is destroyed by contact with antimatter, absolutely rigid. Can be made with any desired configuration of openings (for airlocks, cable lead-throughs, structural attachments) but cannot be altered by any means once completed. The "indestructible" property was backed by a huge warranty, which paid out only once (after which the antimatter clause was instituted and GP closed).