7. In the methane molecule $\mathrm{CH}_{4}$, the four hydrogen atoms are located at the four vertices of a regular tetrahedron with edge length 1, and the carbon atom $C$ is at the center $C_{0}$ of the tetrahedron. Let the four hydrogen atoms be $H_{1}, H_{2}, H_{3}, H_{4}$, 
then $\sum_{1 \leqslant i<j \leqslant 4} \overrightarrow{C_{0} \vec{H}_{i}} \cdot \overrightarrow{C_{0} \vec{H}_{j}}=$ $\qquad$ .
Solve $2 \sum_{1 \leqslant i<j \leqslant 4} \overrightarrow{C_{0} H_{i}} \cdot \overrightarrow{C_{0} H_{j}}=-\frac{3}{2} \Rightarrow \sum_{1 \leqslant i<j \leqslant 4} \overrightarrow{C_{0} \vec{H}_{i}} \cdot \overrightarrow{C_{0} H_{j}}=-\frac{3}{4}$.