The game itself is of little interest. Simonson's blunder (or was it a miscalculation) on move 12 and his awful 13th move gave Kashdan an easy present.

1. e4 e5
2. Nf3 Nc6
3. Bb5 a6
4. Ba4 Nf6
5. 0-0 Be7
6. Qe2

The Worrall Attack, a reasonable alternative to the more usual 6. Re1.

6... b5
7. Bb3 0-0
8. c3 d5

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This line--a cousin of the Marshall Attack after 6. Re1 b5 7. Bb3 0-0 8. c3 d5--is probably better than the "safe" 8...d6.

9. exd5

As <jerseybob> has pointed out, in his crucial last-round victory over Donner at Santa Monica 1966, Spassky here played 9. d3. The comments of Spassky and Donner on the opening are interesting:

"The Dutch grandmaster willingly employs the Marshall attack, Therefore in the present instance I did not want to meet his wishes and take up the challenge." (Spassky)

"Theory says that White cannot accept the pawn, but if he does not he has a good game..."(Donner)

Both moves (9. exd5 and 9. d3) are in fact reasonable, and the choice is primarily one of style.

9... Nxd5
10. Nxe5

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10... Nf4

10...NxN, as in the usual Marshall line after 6. Re1, is probably best. But the text (which sets a fairly transparent trap which Kashdan carefully avoided) need not have led to the catastrophe that followed.

11. Qe4 NxN

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12. d4!

12. QxN(f4) would allow Black to get the better game with 12...Nd3. 12. QxN (e5) loses to 12...Bd6.

Since Kashdan was unlikely to fall for any of this, Simonson should have been prepared for the text, the position after 12. d4 being:

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Black is fine here after the simple and obvious 12...Bb7. INexplicably, Simonson played 12...Nh3? and was instantly lost. Did he forget his Rook was hanging?

13. gxN

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With two pieces en prise, Black must loss material. But he could have limited the damage and emerged with a Rook for two pieces and at least practical chances after 13...Bxh3 14. dxN BxR 15. KxB. Instead, perhaps thinking he had a compensating attack of some sort [he did not], Simonson blundered with:

13... Ng6??
14. QxR

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Simonson had attacking chances here, but nothing remotely worth the Rook Kashdan had just snatched.

14... Bd6
15. f4

Kashdan could also have played 15. Nd2 or 15. Bc2. With his extra Rook, he only had to avoid an accident in defending against Sinonson's coffee-house attack.

15... Qh4

Throwing the kitchen sink into his desperate attack:

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16. Qf3

16. Rf2 would also have sufficed.

16... Bxh3
17. Qg3

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Instead of 17. Rf2 (which also wins), Kashdan offered the exchange to be able to trade Queens and reach an winning endgame a piece ahead. But Simonson, hoping for miracles, needed to keep the Queens on the board to have any chance.

17... Qh5
18. Re1

Kashdan could also have played the nasty 18. Bd1 driving the Black Queen to f5 and pretty much stopping Simonson's attack in its tracks. But it turns out there was no need to do so since Simonson played Qf5 anyway (I expected the last gasp attack with 18...Bg4).

18... Qf5
19. Bc2

Cute:

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19... QxB

Resigning himself to his fate. 19...Qd7 seemed the only way to retain real attacking chances. Exchanges obviously did not aid Simonson's cause.

20. QxB Bxf4

Simonson was apparently now just going through the motions. 20...b4 or 20...Nxf4 were the only ways to keep any semblance of pressure on White.

After 20...Bxf4, the position was:

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21. Na3!

Game over. There were other ways to win, but this was surely the fastest.

21... Qa4
22. BxB NxB
23. Qf3

23. Qd7 would have been more brutal, but the text also left Simonson nothing to play on for.