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\begin{bmatrix} a & 0 & 0 \\ 0 & a & 0 \\ 0 & 0 & a \end{bmatrix}

[/tex]

for simple cubic

[tex]I=

\begin{bmatrix} -\frac{a}{2} & \frac{a}{2} & \frac{a}{2} \\ \frac{a}{2} & -\frac{a}{2} & \frac{a}{2} \\ \frac{a}{2} & \frac{a}{2} & -\frac{a}{2} \end{bmatrix}

[/tex]

for volume centered cubic

[tex]F=

\begin{bmatrix} 0 & \frac{a}{2} & \frac{a}{2} \\ \frac{a}{2} & 0 & \frac{a}{2} \\ \frac{a}{2} & \frac{a}{2} & 0 \end{bmatrix}

[/tex]

for face centered cubic

I don't see any logic for this matrices? How can I get this? I can axcept that simple is P because minimum distance between neighbors is [tex]a[/tex]. But But what with the other two matrices?