  To seek half-metallic ferromagnets compatible with the GeSi semiconductor, we
use state-of-the-art density-functional-theory methods to study cubic
$\mathrm{VGe_3Si_4}$. After optimizing its crystal structure with various
magnetic orders considered, we find the ferromagnetic structure is the ground
state phase and investigate its stability, and then study its spin-resolved
electronic structure with an improved method. Our calculated results show that
the cubic $\mathrm{VGe_3Si_4}$ is a half-metallic ferromagnet with
half-metallic gap 350 meV, and it is structurally stable against deformations
and magnetically robust against antiferromagnetic fluctuations. These results
make us believe that the cubic $\mathrm{VGe_3Si_4}$ could be synthesized as
good thin films soon and used in practical spintronic devices.
