The　optimized　bulk　modulus,　supercell　volume　and　bond　length　were　investigated　by　a　plane-wave　pseudopotential　density　functional　theory　(DFT)　method.　In　order　to　compare　results　of　calculations　and　experiments,　the　Local　Density　Approximation　(LDA)　and　the　Generalized　Gradient　Approximation　(GGA)　methods　were　used.　The　results　show　the　data　calculated　by　GGA　and　LDA　are　fitted　well　with　those　of　the　equation　of　state.　The　calculated　energy　from　LDA　is　larger　than　that　of　GGA,　while　the　supercell　volume　from　LDA　is　smaller　than　that　of　GGA.　Further,　we　found　that　there＇s　a　difference　of　nearly　50　GPa　for　the　calculated　results　between　LDA　and　GGA.　The　bulk　modulus,　supercell　volume,　lattice　parameter,　and　bond　length　calculated　from　LDA　are　more　accurate　than　those　of　GGA.　When　the　coefficient　of　thermal　expansion　is　introduced　to　optimize　the　LDA　method　of　supercell　volume　calculation,　the　result　will　be　in　good　agreement　with　the　experiment.