This bias potential is used to study solid-liquid transitions with the interface pinning method.

Reference:

[Ulf R. Pedersen, J. Chem. Phys. 139, 104102 (2013)]

Usage snip:

fix [fix-name] [groupID] rhoKUmbrella [nx] [ny] [nz] [kappa] [anchor-point]
thermo_style custom step temp pzz pe lz f_umbrella f_umbrella[1] f_umbrella[2] f_umbrella[3]

where the parameters set the harmonic bias potential U=0.5*kappa*(|rho_k|-anchor-point)^2 with the wave-vector elements of rho_k to k_x = (2 pi / L_x) * n_x, k_y = (2 pi / L_y) * n_y and k_z = (2 pi / L_z) * n_z.