Chaos Theory works out of the box, technically. Singleplayer mode at least.
It's a dated game, and it's advised to at the very least use a widescreen fix to perform well on modern computers:
https://fusionfix.io/wfp#scct

In the .ini file, make sure to set your screen rez. There are a few other options there that can be turned on or off, or optionally tweaked like the lights on Sam's suit.

Cool! But is it still possible to play the Coop mode?! I tried a few years ago with a friend and it didn't work out.

Feldo, I just bought this game for my friend. We were able to play no issues using ZeroTier. If you have any questions add me and im happy to talk you through it. currently im facing issues i didnt have of my screen being white or black when nv/therm on, but trying to resolve that now. last i played in 2022 i did not have these issues, and since upgrading to windows 11 that is my only assumption. all other hardware is the same.

Thanks. I used GameRanger and Hamachi a few years ago. Didn't know about ZeroTier. It doesn't seem specifically made for games, I'll look into it!

Some people on this forum seem to have no problem playing. Hope it will end up working!

Limit the FPS to 60. Anything above that will mess up the physics of lights hanging on cables, curtains shaking in the wind etc. Also, if you want to play in widescreen resolutions, you have to apply a widescreen fix, otherwise some screen elements will stretch, like the rifle scope. Here is the widescreen fix: https://fusionfix.io/wfp#scct

Although I must say that I played it through several times without the widescreen fix. It's not THAT critical.

I was having consistent crashing until I applied the widescreen fix mod. Since then, it's worked flawlessly.

weighing in, the game did not work until I got the widescreen mod from pcgamingwiki. Apparently you need to limit fps to 35 or under or there's other issues so I'm doing that using Rivatuner and limiting it to 30. There seems to be no other issues.

The game should work fine out of the box. Maybe there were issues with windowed mode in your case, as the widescreen fix obviously includes a Direct3D 9 wrapper.