How does one obtain these achievements in Black Mesa: Source?

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Quantum Capacitance Overcharge it anyway.

Remote Detonation Kill a Barnacle with a Satchel.

Unified Coupling Theorum Help the Barnacle and Headcrab find true love. [0/10]

Permeable Infrastructure Use the alternative path in the Office Complex.

I'm having trouble with these four in Black Mesa: Source. I have already completed all the other achievements, but these have stumped me.

What exactly is a Barnacle? Do they mean Headcrabs? I'm very confused.