Figure 1: Degrees of freedom in the PN model. a) Atomic rows on one side of slip plane (dashed line) are labeled by index i . Distance between adjacent rows in defect-free crystal is Δ x . b) Displacements of rows with respect to those on other side of slip plane are described by vector ⃗ δ $_{i}$. Figure 2: Fourier series representation of GSF surface on the basal plane of hcp 4 He as modeled by the SWF model. a) GSF energy (in mJ/m 2 ) as a function of two-dimensional displacement ⃗ δ (in units of b p = 1 $_{3}$√ 3 a = 2 . 0996 Å). b) Contour plot of GSF surface reflects symmetry of basal plane of hcp surface . 2 . . . . . . . . N x y z . . . . . . . x y z Δ x i 1 ⃗ δ i 2 N i 1 a) b) 9