The　interaction　of　HCN　and　HNC　with　the　different　sites　of　Cu(100)　surface　is　studied　by　using　density　functional　theory　(DFT).　Cu-14　cluster　is　used　to　simulate　the　surface.　The　present　calculations　show　that　HCN　and　HNC　bind　to　the　surface　in　an　end-on　geometry　with　the　molecular　axis　perpendicular　to　the　surface　and　adsorption　on　top　site　is　prefered　with.　the　adsorption　energy　of　18.5　kJ(.)mol(-1)　and　42.6　kJ(.)mol(-1),　a　weak　adsorption　in　accordance　with　the　experimental　results.　Tilting　either　the　C-N　bond　or　the　N-C　bond　away　from　the　surface　destabilizes　the　system.　After　adsorption,　the　calculated　C-N　stretching　frequencies　are　blue　shifted　for　end-on　bonded　HCN　and　HNC　on　top　of　Cu(100).