
A Computational Verification of Lattice Harmonics
To verify that λ = 17 emerges as a strict dynamical property of the K = 12 FCC lattice rather
than a numerological input, we computationally evaluated the full Born-von Karman phonon
dispersion relation along the [110] face diagonal direction.
The 3 ×3 dynamical matrix is constructed from the 12 nearest-neighbor bond vectors with
central and transverse force constants. The sole geometric input is the K = 12 bond topology.
As shown in Figure 1 and Table 2, the lattice dispersion deviates from the linear continuum by
0.570% (Transverse Acoustic) and 0.803% (Longitudinal Acoustic) exactly at λ = 17.
Figure 1: Phonon dispersion along the [110] face diagonal. The left panel (a) shows the disper-
sion of transverse (TA) and longitudinal (LA) acoustic modes compared to the linear continuum.
The right panel (b) highlights the fractional deviation from the continuum, demonstrating the
transition into the dispersive regime at the coherence boundary λ = 17.
This confirms λ = 17 represents the maximum coherence boundary: waves with λ > 17
propagate coherently with negligible deviation (< 0.5%), whereas waves with λ < 17 enter the
dispersive lattice regime. The standing wave condition on the FCC face diagonal gives a first
harmonic of λ = K
√
2 ≈ 16.97. Discretization to integer lattice spacings yields ⌈K
√
2⌉ = 17,
dynamically confirmed by the continuous lattice spectrum.
Simulation Source Code (Python)
1 impo rt numpy as np
2 fr om sci py . spat i al import cKDT r ee
3 fr om sci py . s parse import l il_matrix
4 fr om sci py . s parse . linal g i mport eigsh
5
6 # 1. FCC L attice Definiti o n
7 NN = np . arr ay ([
8 [1 ,1 ,0] ,[1 , -1 ,0] ,[ -1 ,1 ,0] ,[ -1 , -1 ,0] ,
9 [1 ,0 ,1] ,[1 ,0 , -1] ,[ -1 ,0 ,1] ,[ -1 ,0 , -1] ,
10 [0 ,1 ,1] ,[0 ,1 , -1] ,[0 , -1 ,1] ,[0 , -1 , -1]
11 ], dtyp e = float )
12
13 # 2. Dynamica l Mat rix
14 def d y n _ matrix (k , alp ha =1.0 , be ta =0.3 ) :
15 D = np . z eros ((3 , 3))
16 for n in NN :
17 c = 1.0 - np . cos ( np. dot (k , n) )
18 nh = n / np . l inalg . norm ( n )
19 D += alp ha * np . outer (nh , nh ) * c
20 D += be ta * (np . eye (3) - np . out er ( nh , nh)) * c
21 return D
22
23 # 3. [11 0] Face Diagonal Dispersi o n
6