It's actually -20 since it's just too light. With base catchvalue of ~4.7 that theoretically boils down to -16 CatchValue (ie: impossible).
That said, there seems to be some evidence of it not allowing you to go below 0, but we don't know what value it caps at. Heavy Ball Beldum could be anywhere between 0.01% catchrate or lower.
I'm fairly sure abilities and held items don't affect the Heavy Ball's catch rate.
Beast ball doesn't actually make the CatchValue negative, though. Only Heavy Ball does this.
I used an (injected but legal post-Poké Bank) Skill Swap Harvest Exeggutor and an A-Marowak to keep the Beldum alive indefinitely. I caught the Beast Ball Beldum is ~75 balls. By comparison, I chucked 300 Heavy Ball and it didn't even shake once.
I've seen various values. All the online catchrate calculators actually give impossible values for the Heavy Ball.
Hey for whatever it's worth, I was tracking this conversation with interest earlier. Tonight while rewatching Friends, I tried catching a Komala (43.9 lbs / 19.9 kg) in a Heavy Ball while I had the time to button mash. It didn't click for a few throws.But we do:
CatchValue = ((( 3 * Max HP - 2 * HP ) * (Catch Rate * Ball Modifier ) / (3 * Max HP) ) * Status Modifier
Catch = 1048560 / √(√(16711680 / CatchValue)) = (2^20 - 2^4) / √(√((2^24 - 2^16) / CatchValue))
Source
We're just not quite sure what happens when the CatchValue actually goes in the negative and what the floor would be.
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Just ran some tests:
Lv.27 Beldum with 1/66 HP, Paralysed:
Beast Ball: CatchValue = (((3*66-2*1)*(3*0.1)/(3*66))*1,5) = 0.45
Heavy Ball: CatchValue = (((3*66-2*1)*(3*1)/(3*66))*1,5) -20 = -15.5
Back in RBY and GSC, my brother and I would often eagerly hold down or tap A while throwing poke balls on unlikely catches, hoping to eek out a good roll and nab our treasures more quickly. Tonight I happened to try holding or tapping some combination(s) of A, B, X, Y while throwing Heavy Balls. The fourth attempt or so clicked. Another clicked multiple times. I tried no tapping or superstitious holding = no clicks; I threw a few more with my superstitions gaining traction, however ridiculous.
For reference, Beldum weighs 209.9lbs or 95.2kg according to Bulbapedia. If I understand the earlier mathematical quandary, I want to just confirm that the catch rate doesn't seem to "go negative." If I've missed something key, my apologies!
I don't know if I believe that button mashing during an RNG roll can make the difference in something like a low catch rate ball / Pokemon combination. Nevertheless, I have a Heavy Ball Komala fresh from Route 11. Perhaps it's worth another go on Beldum?