Dimer and SERS hot-spot calculator

Electromagnetically, the SERS enhancement is about |E|⁴/|E₀|⁴: the field enhancement at the laser wavelength times that at the Raman wavelength, which are close for small Raman shifts. Square the |E|²/|E₀|² hot-spot value of this calculator: a gold dimer of 60 nm spheres with a 5 nm gap gives |E|²/|E₀|² ≈ 1,400 in the gap, so SERS ≈ 2 × 10⁶.

Smaller gaps couple the particles more strongly: the gap field grows steeply and the bonding plasmon shifts to longer wavelengths. Polarization along the dimer axis drives the gap mode; across it, the particles hardly interact.

Yes: both spheres' multipole fields are coupled through the exact translation theorems, converged in the number of orders, so the gap field is accurate down to nanometre gaps.