Atomic structure and SiH4-H2 interactions of SiH4(H2)2 from first principles

First-principles density-functional theory (DFT) calculations are used to understand the crystal structure, bonding, and vibrational properties of the recently discovered high-pressure SiH₄(H₂)₂ compound. We find a general decrease in the frequencies of the intramolecular H₂ stretching modes with increasing pressure, where the tetrahedral H₂ exhibit markedly stronger softening than octahedral H₂ . Our DFT results suggest a weakening of the H₂ bond that is explained by increased orbital overlap and electron sharing between the silane and hydrogen molecules, which also account for the unusually high hydrogen capacity of SiH₄(H₂)₂ .